First please forgive my lack of knowledge on these topics but I need to know.
I have a 1953 Oldsmobile engine that I’ve been working on and have discovered the compression ratios have differed between 1949 and 1953 from 7:1 to 8.5:1. My question is; how did they change the compression ratios without changing the bore and stroke. Having only seen the 1953 stock flat-topped pistons I have assumed the earlier pistons were dished to achieve the lower compression ratios. That being said, where can I find pistons that will give me a slightly higher ratio, maybe 9 or 9.5:1?

 

Let me first give some possibilities to changing compression ratio's. "Advertised compression ratio's" are rarely the actual compression ratio. The actual compression ratio is always less than the advertised compression ratio.

Pistons can vary compression ratio by different piston tops.....Dish, flat top, domed up.

Head gasket thickness also helps determine compression ratio. Thinner head gaskets increase compression ratio, thicker decreases compression ratio.

Heads: Combustion chamber volume helps determine compression ratio. Milling material from the combustion chamber side of the heads will increase compression ratio.

Cylinder block: Milling material from the cylinder head mating surface can increase compression ratio.

Any and all of the four listed above determines your compression ratio without changing bore and stroke. You will need to delve deeper into the specs to see what Oldsmobile did to increase compression.
I hope this helps answer your question.