> Here is a mystery to be solved. It looks like the SDS-2 I have is of later
> date and maybe not of Simmons origin after all, but all other stuff
> corresponds to the description so it gets me thinking that the original
> electronic design might have faltered and was replaced.
Have you looked at the bottom side of the voice boards?
It's common to etch a make or model number along with
the copper tracks.
Also, if you cad take a better pic of the top of the board,
I may be able to read date codes off the ICs. Then at least
you'll know if the whole thing is from 1983. There's always
a chance that a better headphone amp died and got replaced
later...
Steve