Giant TCR C2 : lug under top tube

hi all,
just bought a giant tcr c2 and notice there is a recessed female lug on the bottom of the top tube just behind the head tube.
it has some sort of plug in it.
does anyone know what's for?
g

As in a braze-on for a frame mounted pump?

It's a mount for a race number plate. I have a TCR C2 (Sort of...just the frame remains from the original bike. Lots of mods. )

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