In the stock cluster the large printed circuit has electrical connections for the gas gauge and a brake warning light, and it connects them to the large plug on the end of the cluster. I assume that the add-on printed circuit makes the proper connections between the stock circuit and the new tach/gas gauge. You could probably make the connections manually with new wires, but it depends on how handy you are with wiring and reading diagrams.
The connection for the tach on an original cluster is a three wire plug that does not depend on the printed circuit board, so I assume the new circuit is for the gas gauge and warning light connections.
I have a tach cluster in my truck and have found that original tachs are a little flaky sometimes. It appears to be a problem with solder connections that goes bad in the original boards. There are many kits out there to fix the 70's tachs in Corvettes, using a new circuit board on the back of the original tach. However, the board on our trucks is not the same, so you can't fix an OEM truck tach with the Corvette board.
I think there may be other online vendors who sell the tach/gas gauge upgrade, but I'm not positive. LMC is always fairly pricey, but sometimes its the only source for special items.
Bruce