My 1985 Winchester 101 XTR lightweight is stamped '1.300 kg' on the bottom barrel. Does this refer to proof pressure or barrel weight?
I weighed the barrels and they weigh exactly 1.300 kg. I see other marks that the gun was proofed but no other indication to what level of proof.

the 101 has a 2-3/4 [or 3" never seen a 3" 101 though]chamber which both have a saami maximum chamber pressure.
[12,000 PSI, or 1050 BAR airc]
your 101 would have been tested to a higher pressure than that,, probably 80% higher.

it would then get a stamp [symbol of some sort [usually in an oval or a circle] 'saying' it passed proof,,, but not a maximum BAR or PSI type number stamped on them.

The 1.300kg stamp is the barrel weight, as you have correctly surmised.

I beilieve that all 101's are made to SAAMI specs which would easily pass proof in the UK's proof houses.