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The truth is there is little difference between this group of cameras. The S1, S1a, Sv etc were all very, very similar. But the SV was regarded as the top camera as it had a timer for selfies (or whatever they were called in the day).
The similarity between cameras is demonstrated by the fact that two cameras (I think it was the S1 and S1a) were effectively identical. Asahi Pentax wanted to release a cheaper camera to increase their overall market share by tapping a different demographic. The original camera had 1/1000th maximum speed so they issued a new camera with a maximum of 1/500th to provide an excuse to sell it more cheaply without destroying the market for the dearer camera. But in truth, the only thing that was changed was the top plate - the top speed marking was left off but the mechanics for the faster speed was still present in the cheaper camera and could be accessed simply by moving the shutter speed dial to the unmarked position beyond 1/500th. I know it is true because I owned both (I am just too lazy to get off my bum to check which were the models I am referring to 🙂 ) I like that story.
This is true to an extent.
The S1 was part of the 2nd generation of the the original Pentax cameras, while the S1a was part of the 3rd generation. The main difference between the 2nd and 3rd generation was the automatically setting frame counter, and the film door latch is different.
The S3(H3) and S1(H1) were essentially identical cameras, but the S1(H1) lacks the markings for the 1/1000 shutter speed. Mechanically, the position is still there and useable, although I have read that from the factory this unmarked position was not adjusted to an accurate 1/1000 speed.
The S1a, SV, and S2 Super (originally only sold in Japan, and is marked S2) were the last of the S series bodies. The SV was the top model, the S2 Super was the same camera minus the self timer, and the S1a was basically an S2 Super minus the 1000 marking on the shutter speed dial.
Though I have never used one, I agree that the Pentax AP, S, and K are the best looking of all the Pentax bodies. Functionality wise, I think the SV is maybe a bit better just for the single (non rotating) shutter speed dial, it has (supposedly) a better focus screen, and it has the automatic frame counter.
I also like the diaphragm cocking lever on the early Auto Takumar lenses
🙂