Hello, I have been searching the forum and the world wide google to no avail on this specific 77 alarm question. Hopefully one of you friendly fellow C3'ers can help a newbie out.
My new to me 77 has both a front fender mounted alarm lock and a wired drivers door key cylinder. Everything I have read and found online says it should be one or the other, not both.
I have the drivers door panel off to replace the pawl that connects the key cylinder to the lock arm mechanism and low and behold, there is an alarm switch and ant-tamper switch that slipped off the key lock cylinder. It looks original to me but does appear to work. It fell off when the cylinder pawl fell off, but sliding it back on and working the door key ignites some clicks in the cabin when turning the door key to lock/unlock.
My replacement pawl from Zip is supposed to be delivered today so I am excited to put it back together and see what I have, but is it possible I may have a franken-vette with a 76 or earlier front left clip on it that still had the fender lock?...or am I missing something entirely on the whole alarm lock changeroo thing that happened in 77?
The fender lock switch appears to be wired up as well but I haven't been able to fully trace the wires back.

 

That's a strange one all right. I have wiring schematics for both early and late 77's. But not mixed.
perhaps there are some other clues that could help you narrow it down.
Early 77 has one fusible link off the starter solenoid. Late has 2. What is your vin? Build date off of trim plate on A piller?
perhaps way back when the car was new or fairly new the owner of a early 77 heard that the later 77's had the alarm switch in the door. Which is much nicer than the remote switch in the fender. So he went back to the dealer and asked to have his car fitted up with the upgrade parts.
Highly likely.
Changing the front clip much less likely. Wouldn't you just fill the hole in that case?

 

From this drawing there is also a difference in the wire colors between the 1st design on the left and the 2nd design on the right.

 

Dang 4 Vettes, now that is a logical sequence of events I can see happening and probably was the case here.

I found the alarm horn in the front drivers fender by the firewall. I also saw mounting holes for the rear horn above the rear drivers tire/muffler but the horn was not there.
I confirmed the front fender lock mechanism is also wired but I lost track of the wires when they went into a plastic cable harness and I just ran out of energy for it. Ill track it down later and figure out how that works.
The front fender alarm lock is not a regular key like the door and ignition keys. It is one of those round, slotted lock things like you would see on a bicycle u bolt lock. Is that an indicator of an aftermarket install?

So after a 2 day, 10 hour weekend marathon of trying to re-assemble the lock/alarm mechanisms inside the door, I have a tip to share with anyone needing to do the same- tie a piece of sewing thread to the door lock pawl and to the pawl retaining clip and tape the middle of the string to the top of the door so when you drop these pieces inside the door (I did at least 25 times before I wised up) you can easily grab them from the string ends and try again. A pair of long, bent tip hemostats and a long flat blade screw driver were the tools that got it done for me. Getting that retaining clip in while keeping the pawl and alarm switch in place on the door lock cylinder was a challenge since I couldn't get a hand in there. The retaining clip has a bent in nub to lock onto the pawl so it wouldn't just gracefully slide in place. A combo of screwdriver pressure sliding the far end of the clip to lock it in and lifting the nub end was what finally got it in place and locked in.
Once I got the lock pawl and alarm mechanism secured inside the door, I tested the door key with the door open and the alarm went off and on just like originally designed! A fairly primitive alarm and super annoying horn sound but really cool that almost 50 year old technology still worked.

 

Sounds like you got it working! Great!
I have mine working as well but rarely actually use it.
The fender alarm switch uses the same key as the door from factory. So your round key switch is indeed a replacement.