Are the left handle bar control module (light switch, horn button etc.) interchangeable between the early and late C10's? I know the buttons have a different look but can I swap out the whole thing for a latter model?

IIRC they are interchangeable, but not exactly plug and play.  I cannot remember the specifics at the moment; I'm sure somebody will be by shortly

Looking at the wiring diagram, it looks to be plug-and-play.  With one possible exception.  There is an extra wire on the earlier bikes that is part of the "Reserve Lighting Device" circuitry.  I think all that will happen using the newer switches on an older bike is that the RLD won't work anymore.

Assuming nothing has been rewired on the bike, all the wires going through the connector should be the same color combinations on each side of the connector.  (Easy enough to check before you even plug it in.)  You'll just have a "spare" BLUE/ORANGE wire on the bike side when you get done.

Since the positive voltage for the headlight goes through the RLD on the early bikes, you may have to remove and bypass it.  That is accomplished by removing the RLD and putting a jumper between the BLUE and the BLUE/ORANGE wires on the connector.

Looking at the wiring diagram, it looks to be plug-and-play.  With one possible exception.  There is an extra wire on the earlier bikes that is part of the "Reserve Lighting Device" circuitry.  I think all that will happen using the newer switches on an older bike is that the RLD won't work anymore.

Assuming nothing has been rewired on the bike, all the wires going through the connector should be the same color combinations on each side of the connector.  (Easy enough to check before you even plug it in.)  You'll just have a "spare" BLUE/ORANGE wire on the bike side when you get done.

Since the positive voltage for the headlight goes through the RLD on the early bikes, you may have to remove and bypass it.  That is accomplished by removing the RLD and putting a jumper between the BLUE and the BLUE/ORANGE wires on the connector.

Seems to me I remember Jimmy fighting this problem when he put a newer switch assembly on the 86 project bike.

Thanks for the replies