Ah, another one of my ex allen attachments!!
Right. It is for a Model F machine, BUT you have the depth wheel missing.
Looks like its been adapted in the past to fit another machine. Not an Allen. That plate will just bend if used in anger. The bars which that plate it bolted to are not long enough for a model F.
Bit of a bitza. Never noticed it when I had it. Came with a job lot, which in turn sold to Ade, the guy you brought it from!