1951 Capex Souvenir Card

I came across this earlier when looking through my first day covers it was inside one of the covers.
It looks like there was a stamp of some sort attached to it at some time and I'm thinking it may have been a Cinderella stamp that was made for this the first Capex in 1951.
I bought a shoe box that was full of loose stamps and about twenty first day covers so it may have fallen off and be in with the loose stamps.
Anyone come across one of these before or know what stamp was attached to it?

Cheers, Bill

Yes, most possibly a cinderella, not postage, no address.
I have no cinderella that meets that format.

Using the same Mercer cachet, but on a card, with the words "First Day of Issue" at the lower left, it is clear the intent is to apply postage and not any Cinderella. I would expect to see these with various frankings of Unitrade 311-314. I view this as an unserviced cacheted card.

Makes sense,
Nicely solved John.

I do believe it had a stamp affixed to it by hinge, and stamp later removed or fell off and it sure looks like hinge gum remains.

This is not made with a card stock it's actually regular paper maybe it was included inside one of the covers being sold at the exhibition. A bit of an oddity not a post card or a cover.
There was something attached and the box only fits with Scott #310, #311 and #312 so I took a picture with the #312 to add some color.
By the way it measures 6" X 31/2".
Thanks to all.

Cheers, Bill