That is very close to 1/4" = 6.35mm so we can assume that is what the builder was aiming for.
Depending on how far the curved expansion links can move you may or maynot get the same movement at the valve rod.
The four ctr drill holes suggest one for each eccentric, not sure if they would have been used to turn them insitue or whether they were used to clock the eccentrics in once fitted to the crankshaft.
Can you take a Sharpie or similar and mark the position of the two crank pins on the flange, it looks like one is at about 4 o'clock and the other at 8 o'clock