Digit stamp on KR heads...
It refers to the various versions... 1 in 52 till 7 in about 67. Most variations are on the angled shelf...
And not all KR heads are stamped, so it's a real muddle...
So, nothing to do with compression ratios.
I seem to remember the last version, 8, is the special ""shelfless"" hear for the dome pistons.
As the shelf is a necessity on a side valve to reverse the flow properly, it is generally assumed that #8 is not the best set up despite the rise in CR.
Reiman's 149mph average qualifying run around the Daytona bowl in 1968 was done after his father swapped the dome engine for Roger's twin Tillies flat track engine with "7" heads and flat pistons
Patrick