...Forget it. Mods please delete this thread.

IIRC they were usually applied over the finished frame, though I'm sure there were exceptions.

if i were you, i'd eschew whatever was correct for the period and just put them under clear coat (professionally done if possible).

and if you're too lazy/cheap: get some quality helicopter tape/thick archival tape, and make a decal cover sticker.

or you could just get a sticker of a pigeon from a kids store and put it on. but hey - it's your bike.

(EDIT)

i was being serious about the pigeon stickers. it'd be hilarious. i am so sorry if you took it the wrong way.