OK. It's faint enough in the versions I'm looking at that I can't tell if it's supposed to be "358" or "633" or maybe something else, even with glasses. Even if it is 358, it's set off by itself, and not associated with the C20-T750 order code the way the other identifiers like the control number, item description, and serial number are. As such, it still doesn't give any real credence to the idea that it's some sort of extension to the catalog number. If Klein's really thought that the 36" and 40" rifles were different things, they wouldn't have had the two sizes sharing the same order number. And, if Klein's didn't have any 36" rifles available, "358" is meaningless as some kind of disambiguator. They may have simply fulfilled the order with what they had, figuring that the people buying these things weren't liable to be the type to be picky about the exact length. They just wanted a serviceable rifle cheap, cheap, cheap, and there would be few complaints. Those who did object could then be given the choice of sending it back for a refund or exchange with a 36" model, but they not be many. I've seen this kind of thinking on both ends of the exchange before.
The bottom line is, if Kleins were concerned about sending out two different sized Carcano's they would call one C20-T750 and the other one perhaps C20-T751.
Also worth noting is Kleins were advertising the 36 inch Carcano in Guns Magazine as C20-T50 until July63 which directly conflicted with American Rifleman advertising the 40 inch model from April63 to July63 as C20-T750, which indicates that they didn't give a Royal stuff.
Warehouse staff are not designated to think, they receive an order and go to the appropriate shelf and fill out the order with the given Item Number. To believe that they have to cross reference product item numbers with department numbers across a number differing magazines across a number of months across hundreds of various sporting items just doesn't work in the real World.
The Warehouse staff at Kleins, like Oswald in the Depository would be given orders on a clipboard and would go away and fill them. The following crop of Waldman 7 is all they would be concerned with, which they would write in the serial numbers by hand and then take the product and paperwork to despatch.
JohnM