It's an old city license. The adhesive appears to be similar to what I've seen on automobile license tabs. It's pretty large, - about 2.5 inches tall and fitting 3/4 of the way around the seat tube.
I'd like to remove it without trashing the paint which is in pretty good condition.
There's also a sticker from the bike shop it was purchased from. It's a bit ragged but on the other hand it's an old and respected bike shop and being a vintage 70's bike, I'm tempted to leave that one.