That'a a High Wall breech block, isn't it? Perhaps it is a cut down High Wall Action like some of the "Winders". It would then be a large shank action, wouldn't it?
There is no such thing as an exclusively highwall block. Nor do all highwalls have large shank threading. All thin tang 1885s are low walls and are thread for small shank barrels but the converse is not true. Thick tang low walls are effectively cutdown highwalls but can have small shanks and any block. For instance this one has a scalloped block and a large shank #2 barrel:
Further, these rifles are lettered by their serial number, of course. The serial number is on the trigger bar and the trigger bars can be swapped around among any other similiar thick/thin tang action be it high or low. So what that guns letters to, only tells you how the trigger bar left the factory. And the letter does not denote low or highwall. They were all The 1885 Singleshot to Winchester.
But maybe it did leave the factory a .38-56. I will would not shoot it. Nor would 90% of the people that compete with bpcr Winchesters and know quite a bit about them.