Are the 336, 444, and 1895 different frame sizes? If not why the different model numbers?

Same frame size. Different # for each cartridge. Also the ejection port is milled a little differently in the rear to clear the bigger shells. May also be internal differences.

Same receiver, just some machining differences for the various cartridges. Just like Dillonbuck said, the different model numbers denote chambering in regards to the big bores.

Thanks for the replies.