From David Edgington's most excellent and eminently useful book, The Lister CS Story, page 11:
David is quoting some notes Arthur Freeman Sanders wrote in 1931, about a year into production of the CS line of engines.
Arthur Freeman Sanders was the chief development engineer behind the CS line of engines at Dursley (and was for sure a cool frood who really knew where his towel was at.)
Quoth:
"The auxiliary combustion chamber has a cubic capacity which, as previously stated (page 10),
gives a difference between compression ratio of 19:1 and 15:1. The higher ratio
being sufficient for immediate starting while the lower, being the best running ratio
for a fairly large range of fuels. That combustion takes place in this chamber is evident
by the swirl marks, yet superimposing indicator diagrams taken from both chambers at
the same time; show no evidence of a time lag, the diagrams being identical."
Looking at the drawings in the book, it's evident that the plug which replaces the COV reduces the combustion chamber to the same volume as one with the COV screwed fully in.
I think it was George Breckenridge that answered my question about that in about 2005 to the effect that the connecting rod was shorter or piston pin height was higher in Indian engines to compensate.
And I seem to recall Dave (XYZER) did some measurements and confirmed the 17:1 compression ratio for the plugged Indian 'roid engine. I myself cced a 6/1 head and found its volume, but I don't recall what that number was.
However, when taking that volume and adding the volume over the crown of the piston, I also came up with 17:1. So with everyone in violent agreement, the issue was considered to be settled.
[Edit: Page 35 of David's book talks about the 8-1. It says,
"The salient feature of the 8-1 must surely be the cylinder head which does away with the Freeman-Sanders change-over valve in favour of just a single combustion chamber. A plug is used to seal off the starting chamber (see illustration) from the combustion chamber proper, this plug being enclosed in position by screw-in plug 008-03179. [The] revised combustion chamber size reduces the compression ratio from 19:1 to 17.5:1 for both start and run conditions. Starting is effected in the normal manner but without any means of adjusting the compression ratio."
So there you have it. 17.5:1 compression ratio, at least with the 8-1. And I know we confirmed the compression ratio of the Rajkot 6/1 engines to be the same when set up properly. ]
Quinn