I should have said MKI or original series. The pistols in our pictures are of the Mark I series. I overlooked, the 1 of 5000 on your barrel. As you say, Ruger never produced a stainless T512, T514 or T678 (Mark I).
The pistols in our pictures are not of the Mark I series. They are of the Standard series.
All Mark I pistols are Target Pistols. The "Mark I series", would be used in reference to the Target pistols only.
The unintentional use of Mark I to group all first generation Ruger automatic pistols into one group is prevalent although profoundly incorrect.
The first and original Ruger auto pistols was produced in 1949 and were named the "Ruger .22 Cal Long Rifle Automatic Pistol" (aka Standard) and was stamped and marketed as such. The newer Target model came out in 1951 with "MARK I" added to the previous stamping on the left side of the receiver.
When the Mark I Target model was released, the use of "Standard" was used for the earlier original models. This became necessary for record keeping plus markings for boxes and catalogs. The Standard series/models, are RST-4 and RST-6, and Target series/models are T512, T514, and T678 (as you posted)
Sorry if I seem a bit pretentious, I know we're on the same page for the most part. But me being a big fan of early Standards, the grouping of ALL first generation automatics as MARK I, MK 1 or a series there of, is a pet peeve of mine.