Sometimes the Brits put the part numbers in the dish as well. Just depends upon the supplier of the pistons. There might be a single number in the dish (like the "2" shown in the top picture) as well which indicates factory adjustment to standard. There would be a number on the block face near the tall stud hole. Early BMC engines went up to "4". Never saw one that way. Biggest I've ever come across was a 3. BLMC later went to letters (A, B) and only two sizes.
From your pictures, I can tell you those are the 11.15cc (nominal) pistons. Hence they are for the 8.8/1 engine. The "dishier" 8.0's have a thinner ridge to the edge of the piston crown.
Now you need to turn the engine to TDC and measure the negative deck. Dial calipers will do it quickly. Measure at the ends of the piston where the pin is located. This will negate piston rock. Average the two readings. If you don't have a dial caliper then use stacked feeler gauge blades and your finger to rub across the gap until you feel like the stacked blades and block surface are equal. Finger tips can feel about 0.001 - 0.002" difference. That's accurate enough.
Then you have to know the cc's in the head. That is the next biggest influence on compression. A clear plastic plate sealed with grease like Alan suggested will tell you the volume of the head. Post the numbers for the deck, the gasket you are using and the cylinder head volume and I'll do the mental math and a pretty close compression number.
That pitting is of no consequence. Seen it many times and worse. The crown is plenty thick. Just make sure there are no sharp edges on the piston circumference to create hot spots and sensitivity to detonation.
BTW, #2 has a nice ring ridge on the thrust side of the cylinder
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Mike Miller