Thanks Bryan. Another question....
Can you check the wire dia of the spring for me, it looks like 1.2mm... What is an RCH spring... excuse my ignorance.
BDA answered the second question. LOL. Common engineering term, considering a yard (IIRC) was the length between the Royal nose and upturned thumb of some monarch or the like, so it's a consideration that most imperial measurements were based on their highness' anatomical figures...
In answer to the second question, I measured it at 1.4 mm. give or take. I don't think it's necessarily critical; the stiffer the spring, the more force to get 5th, so, with no way to measure rate, it may be a case of assemble a sample of different rate springs of that length and have a go.