Oh, I didn't mean that there aren't any $2 FRNs. What I meant was when the Legal Tender vanished, there were already FRNs in all other denominations, just not $1 and $2. While $1 FRN was issued immediately, it wasn't until 1976 that $2 FRN came out, which was actually sold as a novelty item for most part, almost like the Presidential Dollar coins these days. The big gap of about 12 years made people used to not seeing a $2, so much that most people think they never printed them any more. Even today, that perception persists. IMO, that is what makes $2 not popular.
The question about cash register really was how did they handle 2s during 60s and before, as $2 was circulated fairly well. You response seems to imply that even then, there was no place for the 2 in the drawers, is that really true?