Dropping in a round would be easiest; a chamber cast would be more bother. I'll check and see if I have any empty .38 S&W cases - easily mailable.
The revolver looks to be decent quality, from the latter part of the 19th century. Can't read what is on the barrel. There is an inscription on the backstrap. Is it any help?
Make sure its a resized brass case you send him. Brass that was fired in another gun is not the way to check what cartridge this gun could be.
Looking at that gun it should be a metric cartridge like 9mmm but someone mighta set it up for 38 S&W.
If Fucha wants a casing, I'll pull a bullet and deprime. It the photo, the barrel has "38" engraved.
You've seen a lot of antique guns in your line of work, ever see anything like this?



























