Around the US I know there are a bunch of tower/approach freq's and MOA freq's around 121mhz. Most have matching UHF freq's also. I wouldn't think the 121.5 would be used on VHF AM, but, a similar one, maybe 121.650 would have jogged your memory. I think that happens to be a Midway (IL) Airport freq.
(Not to be confused with Midway, a bad memory for the imperial navy)