i have been using a round dial gauge that has the letter M inside a red oblong circle.
it says : "precision" and Made in U.S.A. on the face which is black with white letters and pointer. it reads in .5 lb increments to 60lbs, but i don't think the .5lb scale would be particularly accurate, considering the 2"[or so] dial face and the 60lb range.
i bought several of these at the speed shop on main street way back in the mid 70's.
got three or four for me, and one each for my brother and old man. they came with a rubber "bootie" you could install or not.
all of the ones i own show the same reading on whatever tire i check, so at least they may have been calibrated using the same method when built.