with a 1.66 compression height. It will give you around 8:1 with a 72cc head and a 390 crank. It will end up down in the hole quite a bit too. It could be used with a 428 crank to make a 410 and end up with 10:1 or so.
-20 is the oversize.
You can look it up on SummitRacing.com, just punch 381p in the search box.
It is the one on the end with the longer skirt, note the compression height.
A little closer, and a little out of focus
HTH