Alternator warm with ignition off 1967 C10

[mr.precision]

Bought this truck after sitting for 6 years in a barn. I've got it running pretty well.... 283 and a four speed....but the battery slowly discharges and the alternator is warm to the touch with the ignition off. Amp meter shows a slight discharge when driving. I tested the alternator and it appears good.
Checked all the wires - they look OK.
Voltage regulator is sealed so I cannot get inside to have a look.
2 questions:
What's a good brand regulator to look for? Does it make any difference as to the manufacture? Solid state or mechanical?
Also, what is the device located beneath the existing regulator? It's a cylinder probably 2-1/2" long and maybe 3/4" in diameter. This is a very original '67, so I wouldn't be surprised if it is OEM to the truck.
Thanks in advance.