Ok, But what about this ‘R’?

[Paul Slootheer]

The letter ‘R’ as a testers marking on its own sounds plausible. But two markings, ‘R’ and ‘O’ on one set of (matched) crank cases again makes me wonder…

[Roger Gwynn]

A quick survey around the shed this morning found 9 singles cases
H 5495 model 'T' we know this means 'tested'
S SF with a 'D' in a triangle and 'KC173', which is I suspect later addition
K Nothing
BB Nothing
CC Nothing
P Nothing
CH a VA 'A'
D Nothing
CA Nothing
KL Nothing

[Vincent.vanGinneke]

one more for the record on a 1953 VB case

[simon.holyfield]

Could it be a mark to record some sort of rework or fault rectification?

[paul.jameson]

For the record, I have found:
T on HA prefix cases (1935 250)
T on AC prefix cases (1937 250)
H on BC prefix cases (1937 350

(all marks between the pushrod holes)

but nothing on my 2 sets of AB prefix cases (1936 250) and nothing on my CB prefix cases (1936 500).