It wasn't on this thread, but somewhere on the C&V Forum I seem to remember a BF member saying that they were seeking a 19T cog for a Regina freewheel. Unfortunately I can no longer remember the member's username or find the thread in question.
This eBay source seems to have quite a selection of Regina freewheel cogs. If you're looking for one, that could be a potential source if your usual sources don't pan out.
Disclaimer: I neither know the seller nor have I done business with him (he does have good ratings, though, and seems to have been around for a while). Posted here in case it might help someone.