[kazinator]

This is very similar to a notation occuring in Ronald Rivest's S-Expression proposal from 1996.

https://people.csail.mit.edu/rivest/pubs/RL96.ver-1.0.pdf

He has it as a hexadecimal length preceded by a pound sign (#), a colon, and the raw octet data.

[kazinator]

Right, in later drafts he changed to decimal with no # prefix:

https://www.ietf.org/archive/id/draft-rivest-sexp-04.html