I know basically nothing about proofing, but from threads on here I have no impressions that they have the best interests nor knowledge, so stripping a gun seems unlikely when proofing.

If the sand got there in service one would think it would have been right at the end of its service, as if it were refurbished or attended to by an armourer this sand would have been cleaned away while checking everything else (I don't think there were instructions on accounting for bedding with sand in the fore-end). So perhaps it was in some sandy environment, but right at the end of its life in the battlefield and was never serviced afterwards; or, sometime after being surplussed someone took it somewhere sandy and never bothered to clean it.

The olive paint I have no idea on, but I've seen plenty of 40 - 45 Lithgows with the usual blacking and nothing like olive paint.