Finding data on these devices has proven to be difficult but from memory (which isn't the greatest these days!) I am pretty certain that this device is a sub-miniature relay packaged in a plastic DIL format.
OKI (Japan)
Type: D51C12
Function: Reed relay based electromechanical switch
Often used in the days prior to SS switches where electrical interfaces needed to be isolated - low level current switching only (control signals or even audio)
8 pins DIL plastic case which looks like a standard 16 pin but with more height and of course a number of the pins don't exist!
I have made some quick tests of this device and across two pins I have what seems to be a coil (500 Ohm) and the remaining 6 pins are switch contacts (meaning this is probably a DPDT switch) I used a test supply of 12V DC (draws around 25mA) and had switch changeover occur (as measured at the pins)
New Old Stock
Stored & supplied in suitable safe packaging material
