In our Moss Monaco we found a "electrical" unit with some sort of pendule, all the wires are cut but was still mounted on the bulkhead in the enginebay.

I have the picture in my photoalbum under Electrical unit.

Does anybody know what it is?

looks like an old alarm tilt switch to me

Probably a trembler switch for an alarm.

As it says on the PCB it's a Selmar alarm - they were one of the first company's to market alarms.
Probably from the late 60's/early 70's as IIRC the original ones were round.
They were pretty crude in that despite the adjustment they were prone to be set off by passing traffic as it rocked the car - ask me how I know!
Cheers, Pewe10

had one on my midget early seventies.left it outside a friends all night with the alarm going of in high winds.Went round at 5.00am after a phone call....not popular.

chain and anchor followed and the car went..happy days