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According to the https://www.itu.int/en/pages/default.aspx, International Telecommunications Union's https://www.itu.int/rec/T-REC-X.680/en, X.680 - Abstract Syntax Notation One (ASN.1), the abstract definition for an $(MONO EXTERNAL), after removing the comments in the specification, is as follows:

EXTERNAL ::= [UNIVERSAL 8] SEQUENCE {
            identification CHOICE {
                syntaxes SEQUENCE {
                    abstract OBJECT IDENTIFIER,
                    transfer OBJECT IDENTIFIER },
                syntax OBJECT IDENTIFIER,
                presentation-context-id INTEGER,
                context-negotiation SEQUENCE {
                    presentation-context-id INTEGER,
                    transfer-syntax OBJECT IDENTIFIER },
                transfer-syntax OBJECT IDENTIFIER,
                fixed NULL },
            data-value-descriptor ObjectDescriptor OPTIONAL,
            data-value OCTET STRING }
                ( WITH COMPONENTS {
                    ... ,
                    identification ( WITH COMPONENTS {
                        ... ,
                        syntaxes ABSENT,
                        transfer-syntax ABSENT,
                        fixed ABSENT } ) } )

Note that the abstract syntax resembles that of $(MONO EmbeddedPDV) and $(MONO CharacterString), except with a $(MONO WITH COMPONENTS) constraint that removes some of our choices of $(MONO identification). As can be seen on page 303 of Olivier Dubuisson's http://www.oss.com/asn1/resources/books-whitepapers-pubs/dubuisson-asn1-book.PDF, ASN.1: Communication Between Heterogeneous Systems, after applying the $(MONO WITH COMPONENTS) constraint, our reduced syntax becomes:

EXTERNAL ::= [UNIVERSAL 8] IMPLICIT SEQUENCE {
            identification CHOICE {
                syntax OBJECT IDENTIFIER,
                presentation-context-id INTEGER,
                context-negotiation SEQUENCE {
                    presentation-context-id INTEGER,
                    transfer-syntax OBJECT IDENTIFIER } },
            data-value-descriptor ObjectDescriptor OPTIONAL,
            data-value OCTET STRING }

But, according to the https://www.itu.int/en/pages/default.aspx, International Telecommunications Union's http://www.itu.int/rec/T-REC-X.690/en, X.690 - ASN.1 encoding rules, section 8.18, when encoded using Basic Encoding Rules (BER), is encoded as follows, for compatibility reasons:

EXTERNAL ::= [UNIVERSAL 8] IMPLICIT SEQUENCE {
            direct-reference  OBJECT IDENTIFIER OPTIONAL,
            indirect-reference  INTEGER OPTIONAL,
            data-value-descriptor  ObjectDescriptor  OPTIONAL,
            encoding  CHOICE {
                single-ASN1-type  [0] ANY,
                octet-aligned     [1] IMPLICIT OCTET STRING,
                arbitrary         [2] IMPLICIT BIT STRING } }

The definition above is the pre-1994 definition of $(MONO EXTERNAL). The $(MONO syntax) field of the post-1994 definition maps to the $(MONO direct-reference) field of the pre-1994 definition. The $(MONO presentation-context-id) field of the post-1994 definition maps to the $(MONO indirect-reference) field of the pre-1994 definition. If $(MONO context-negotiation) is used, per the abstract syntax, then the $(MONO presentation-context-id) field of the $(MONO context-negotiation) $(MONO SEQUENCE) in the post-1994 definition maps to the $(MONO indirect-reference) field of the pre-1994 definition, and the $(MONO transfer-syntax) field of the $(MONO context-negotiation) $(MONO SEQUENCE) maps to the $(MONO direct-reference) field of the pre-1994 definition.

The following additional constraints are applied to the abstract syntax when using Canonical Encoding Rules or Distinguished Encoding Rules, which are also defined in the https://www.itu.int/en/pages/default.aspx, International Telecommunications Union's http://www.itu.int/rec/T-REC-X.690/en, X.690 - ASN.1 encoding rules:

EXTERNAL ( WITH COMPONENTS {
            ... ,
            identification ( WITH COMPONENTS {
                ... ,
                presentation-context-id ABSENT,
                context-negotiation ABSENT } ) } )

The stated purpose of the constraints shown above is to restrict the use of the $(MONO presentation-context-id), either by itself or within the $(MONO context-negotiation), which makes the following the effective abstract syntax of $(MONO EXTERNAL) when using Canonical Encoding Rules or Distinguished Encoding Rules:

EXTERNAL ::= [UNIVERSAL 8] SEQUENCE {
            identification CHOICE {
                syntaxes SEQUENCE {
                    abstract OBJECT IDENTIFIER,
                    transfer OBJECT IDENTIFIER },
                syntax OBJECT IDENTIFIER,
                presentation-context-id INTEGER,
                context-negotiation SEQUENCE {
                    presentation-context-id INTEGER,
                    transfer-syntax OBJECT IDENTIFIER },
                transfer-syntax OBJECT IDENTIFIER,
                fixed NULL },
            data-value-descriptor ObjectDescriptor OPTIONAL,
            data-value OCTET STRING }
                ( WITH COMPONENTS {
                    ... ,
                    identification ( WITH COMPONENTS {
                        ... ,
                        syntaxes ABSENT,
                        presentation-context-id ABSENT,
                        context-negotiation ABSENT,
                        transfer-syntax ABSENT,
                        fixed ABSENT } ) } )

With the constraints applied, the abstract syntax for $(MONO EXTERNAL)s encoded using Canonical Encoding Rules or Distinguished Encoding Rules becomes:

EXTERNAL ::= [UNIVERSAL 8] SEQUENCE {
            identification CHOICE {
                syntax OBJECT IDENTIFIER },
            data-value-descriptor ObjectDescriptor OPTIONAL,
            data-value OCTET STRING }

Upon removing the $(MONO CHOICE) tag (since you have no choice but to use syntax at this point), the encoding definition when using Canonical Encoding Rules or Distinguished Encoding Rules:

EXTERNAL ::= [UNIVERSAL 8] SEQUENCE {
            syntax OBJECT IDENTIFIER,
            data-value-descriptor ObjectDescriptor OPTIONAL,
            data-value OCTET STRING }