ORIGINAL: Raymac
I have what I think you are looking for....It is OS part no. 23364000 for 40FP and 40FP-S.
It measures .840" X 1.00" backplate gasket too. Nw in package. $5.00 dselivered.
It won't work. I have an OS40 SF which needs a head gasket of .805 or 20.5mm diameter. The OS40 FP gasket is too large. I have found though that a Thunder Tiger 40 Pro has a head gasket of 20.5mm. I've ordered one of those to see if it fits.