Let's take a look at the WRA ledger notation for the Marine order for 500 rifles in July of 1917.
First USMC Order for 500 Rifles 1.jpg
That's the mysterious ledger entry. Let's say we are the clerk making that entry. We don't have a whole lot of room on the ledger, nor do we have a name for the exact mount being ordered, so what do we do? We call a spade a spade - we call it a #2 mount. It is just a ledger entry, not a contract. Who knew a bunch of knuckleheads would get into this huge argument over a ledger entry 100 plus years down the road?
Maybe the actual contract will clarify the issue. I have never seen the actual contract.
My money is on a misunderstanding in nomenclature.