Right then,

from the top;

L is SP stuff;
SP head has small squish (higher compression)
SP barrel is tall (lower compression)

H is R stuff
R head has large squish (lower compression)
R barrel is short (higher compression)

So the SP head, R barrel is the combo for a race bike c/w some kind of wacky fuel to keep it from detonating itself out if existance.

Either R,R or SP,SP is std for a roadbike, I believe they are both exactly the same once togeather.

R head SP barrel would make a low compression setup.

So the front is running too low a compression (in comparison to the rear) and I'd therefore guess the front is not working hard enough, the rear is overworked and has died. You need an SP head on the front, your old std front SP head would be better than the R you've got now.

Matt? Andy? Steve? Any thoughts?

How does the barrel look?

As to why, it's 15 years old, it's had a rebuild or two for sure, if you buy a barrel/piston and head 2nd hand it's of no concern wheather it's R or SP.
I wouldn't even bother asking, just make sure it's right when you bolt it togeather.
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