Souls,
That’s correct, 1.6875-12 Stub Acme is not tabulated in ASME B1.8. It only means that ASME does not consider it as a preferred size. That does not mean that it, or any other size variation, cannot be manufactured if there is a specific need. Nothing illegal about it. Expanding on the previous post by metalonis, just use the formulas to calculate what you want. Section 1.7 of the spec says the same thing.