There are two types of non adjustable caps. They are not interchangeable. One looks like a Bat Masterson derby hat (Like the one in the picture of the previous post) and the other like an Abe Lincoln stove pipe hat. The Abe Lincoln style is the newer of the two and can be used in lieu of the adjustable type one that has the seepage problems with out any issues. But the Derby looking one is used with an older style crankcase that has a steel cage mounted in the crankcase, where the relief valve assembly mounts, to keep the relief valve ball centered on it's seat. You can mount the derby style one on a non caged crankcase, as it will fit fine, but the ball will be allowed to drift sideways during engine operation due to the lack of the centering cage, and you will have weird, unsteady and unreliable oil pressure because the ball can be and often will be misaligned with its seat.
Good Luck,
Mahlon