I saw an old recipe that called for an stp of yeast and salt...all other measurements were standard. What does stp mean?
Could it simply be a typo? -> tsp for teaspoon?
Check if the overall proportions make sense with this, then that's probably what it was ;)
stp, though I would usually see it as STP or stbls or something similar to distinguish it more from tsp or typos, in my experience is Short Tablespoon which was a common US term for how much yeast to use in many recipies. It is the amount of yeast in the individual envelopes of yeast, 1/4 oz or about 2 1/4 teaspoons. Most US home cooks learned with dry measuring cups and spoons rather than scales, so short tablespoon was shorthand for "a little less than a tablespoon" and Heaping Tablespoon was "a little more than a tablespoon" or closer to 4 teaspoons. (Or Scant Tablespoon is another wording.)