Tom,
Took some pictures of 2501R my black S2. It's build date was 10/69 and has the type S1 suspension like yours. As you can see, the drivers side suspension is at full droop and the clearance between the top spring perch and rear A arm is very tight. I measure my spring diameter around 2.5". Apparently that quarter inch makes a world of difference.
The last picture was suppose to show the difference between my early S2 shock and a TCS shock. The TCS shock is 3.5" in diameter.
Joji