Oly - ALL original equipment Carter YF carbs were stamped. A few early aftermarket YF's (mostly for the 216 and 235 Chevrolet) were not stamped, and identified by tag only.
G.M.C. NEVER used a Carter YF on the 270. Starting about 1966, G.M.C. did use a YF on some 250's and 292's.
Without knowing exactly what you have, I am guessing; but you may have one of the early aftermarket units which are unsuitable for either a G.M.C. 270 OR a Chevrolet 261.
Jon.
