The serial number YG2902591 indicates that is was manufactured in July 1978.
NO box, NO manual, NO accessories.
Weight of SPEC-4 is 54.0 LBS, 150 watts per channnel;
whereas its brother the SPEC-2 at 250 watts per channel weighs only 52 LBS.
This is probably because the SPEC-4 has independant power supplys for the left and right channels.
Looking at the fuse you can see that it is a multi-voltage model.
I bought this SPEC-4 used, long ago, and rarely used it.
The last time that I used it was probably 10 years ago.
It is dirty and has some corrosion from the previous owner,
with me it was kept in my house office or in a box.
July 2021 I took it out and connected it to a Variac and slowly increased the voltage;
somewhere around 40VAC it made a loud clatter that increased as the voltage increased;
so I turned it off and will sell it as non-working.
I have downsized from a house and can only use 30 to 50 watts in my townhome.