I can’t give you the exact dates because I don’t have enough Western Catalogs to help that much. However, I can, in the interim, open the date spread. The 170 SPBT bullet was not listed in my earliest catalogs from the 1920s. Unfortunately, my next catalog is September 23, 1939, and it IS listed in that Price List, under product number K1422C. It is still shown under the same product code in the Price List of January 2, 1942. The same product code is shown in the Price List of March 7, 1946, but in that list, it is shown only as a 170 grain SP Super-X, with no mention of whether or not the projectile is of boat tail design.
The last catalog (Price List) I have that shows the K1422C Product Code, and it and all of the ones I have between March 7, 1942 and it, is that of January 16, 1953. The December 15, 1954 Price List shows the 170 grain SP Super-X load, but now the product codes have changed, and the one for that load is 30303. The 30303 load is shown thru my last price list that is exclusively Western Brand, dated January 3, 1961.
I do not know from any personal experience whether or not the 170 grain SP Super-X bullet remained a boattail after they stopped showing that feature in the price list. One would think that if they changed the bullet design, they would have changed the product code, but that may not be the case.
Sorry I can’t be more help.