Onion address encoding
As you suspect, it's a concatenation symbol. Some times it's $|$ and others $||$.  Or $+$. Python and PL/SQL examples to confirm. Or a cryptographic example: bottom of page 5 of FIPS PUB 202: SHA-3 Standard: Permutation-Based Hash and
Extendable-Output Functions.  

If it were bitwise OR, you'd make a real mess of the public key's use with PUBKEY | CHECKSUM | VERSION.  

It can get weird though if you dig deeper as: Is there a common symbol for concatenating two (finite) sequences?