It a 1275 for sure, and that a smog area head, which makes it a 12G1316, so the mystery is pretty much over, as mentioned earlier, no A-series except the 1275 was ever produced void lifter cover on the block. From there basicly all inline 1275 engines are the same, in the early 1275s the was a block is solid under the center main webbing in the block, and these block most often had the coveted EN40B crank, but the other block only have a small pie shaped opening in the center main webbing, and had the EN16T crank, the same crank, just tuffrited intead of nitrited and made of EN16 sreel rather than EN40 steel.
Side note: Some 1275 engines do in fact have mechanical fuel pump blanking plate, Ray's engine, that I'm building right now is a late 72-73 block and it has mech. fuel pump blanking plate, on Rays motor I will be using the earlier pre smog head, 12G940 so we keep the "earlier feel" to the entire 1963 "project Spridget".
Some of the early 1275s were higher compression from the factory, but that's no big deal if you are rebuilding, you can use the AE21253 pistons with it's higher compression height and make it a higher compression engine for the cost of a set of pistons, cheap horsepower. With just the addition of the AE21253 pistons you can get a 1275's comprtession ratio to around 9.5 to 1 with a virgin block and head deck.
The great thing about a 1275 is you have so many choices in aftermarket goodies, you can go hog wild like we are doing with Ray's motor or you can pick and choose the best bang for the buck, and get good improvement rather cheaply.