Pietta seems to be well into making "fantasy" revolvers, that were never made during the 19th Century. They're is nothing wrong in doing this except it may sometimes cause some confusion.
Colt's
1851 Navy Model was the basis the 1860 Army. The only difference between the two frames was that the 1851 and 1861 models were made with non-rebated cylinders chambered in .36 (actually .37) caliber, and the frames had unaltered water tables.
The 1860 had rebated cylinders, modified water table, and was exclusively chambered in .44 (actually .45).
Remington also had two frame sizes with straight (not rebated) cylinders, with the smaller being chambered in .36 and the larger - and more common - New Army model in .44 (actually .45).
So far it seems that Pietta seems to have made all of their Remington revolvers on the larger frame, and cylinder, but chambered in either .36 or 44 depending on the particular (more or less) model.
As a consequence I have a New Model Army - imported by Lyman - that is a .36 caliber.
With the arrival of fantasy guns you may encounter anything.
As a side note: If someone wanted to make up an especially accurate New Model Army they could buy a .36 cylinder and have it line-bored to match a custom .45 barrel for less money then importing one of the extra high priced European-made target-grade guns.