I never saw it done back then, but it could be. It could also be a manufacturer's logo.
Looking at the whole pic again, I see the same marking on the lower frame tube. Looking at the details of the bike itself, it looks like this was a fairly upscale model. I tend towards it being the manufacturer's logo.
Looking at the whole pic again, I see the same marking on the lower frame tube. Looking at the details of the bike itself, it looks like this was a fairly upscale model. I tend towards it being the manufacturer's logo.