Background info:
When the NES was first released in 1985, Nintendo did not have the manufacturing capibility set up to make large numbers of NES 72-pin boards.
Therefore, many of the first batch of NES games made in 1985 were, in fact, the 60-pin famicom version. Nintendo put a large 60 pin - 72 pin adaptor directly into the NES case. The games looked just like regular NES cases, but wieghed significantly more.
Excitebike, Gyromite, Duck Hunt and Super Mario Bros. Are all believed to have a version with this adaptor board in it. Gyromite and Excitebike seem to be the most common.
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Anyway, I have one of these carts for sale. It is a 5-screw Gyromite. I can provide pictures if nessisary. I'm looking for $20 shipped.