mbatten,
I believe the cartridge is based on the 38 Long Colt, which is a little shorter than 38 S&W Special but since the chamber was reamed for a heeled bullet it will chamber a 38 Special. Starline markets 38 Long Colt cases, but you can just shorten 38 Special. We see rifles of this type with different names, even Holland and Holland. It was likely made In Birmingham "For the Trade", and being unmarked, if the seller was a "Name" company they would have their name applied. If the seller was a local (small) shop, they may or may not mark it. It may have come from the same shop as my friend's Holland and Holland, but likely didn't cost you as much. There are enough of these rifles around that you might take craigd's suggestion and call Accurate Molds; they may have already made a mold that will work for you.
Mike
