How did you go with this one Scott? According to Jeff Holly's book, 'the serial numbers on the early engines were stamped on the nameplate only. Later types had the number on the crank case cover, timing gear guard and the crank shaft ends.' He is referring to not just the OFG, but the OHG, OCG and OGG engines.
In the book is a copy of the instructions for the engines, but it appears to be indentical to the scans already shown in this thread. If the book may be of any use, I'll bring it around one day for you to browse over a cuppa and a biscuit.
I'm not sure if Jeff's book is still available. I got mine from the author in 2004, but I think Plough Book Sales may have had some stock. It's called Bagshaw Engines, but features lots of their machinery. ISBN 0 646 02157 5.