Lady that currently owns the bottle claims that what you think might be 51, she said is IS, looks more like 31 to me. Also, I know for sure that the bottle would fall between the late 60's and the late 70's.According to my book, that mark (AHK),was used from 1944, at least until the 1970s, If that is a 51 on the right side, that may be the year of manufacture.