Welcome to the site. It's probably a 1968 bottle and that was the final year for that logo to be used by Knox Glass. Because they got sold to Glass Containers Corporation. I have no idea what is in the bottle.
I don't think that's a date code, the bottle looks too early for 1968 to me. I'd guess it dates to the 30s, but no clue what the cap said. If you want to find out, soaking the cap in lemon juice might remove enough of the rust to be able to read it. I don't know if you'll be able to remove it or not, it depends on how badly corroded it is.