That's one of the approximately 50 early floorpan Manxes which were produced between approximately November 65 and Summer 1966.

Here is a copy of a letter dated November 1965 signed by Bruce which says they've suspended production of the monocoque type and begun producing your early floor pan type.

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When they made your Manx they only had one mold. After the August 66 issue of Hot Rod magazine came out(which would have been NLT June) with the Manx on the cover, they had an overnight back order of hundreds of bodies. The first thing they did was made additional molds to increase production capacity, but they also made the changes between your style floorpan Manx and the later ones, including the redesign of the rear license mount.

Another thing unique to your type of early floorpan Manx is the windshield frame which was hand made from flat aluminum bar instead of an extrusion. I'm not sure why you're asking though, I'm pretty sure you know this stuff as well or better than I do.