[sdiy] OT: Rock amp question

[John Alex Hvidlykke]

Another, slightly off-topic, amplifier question, that I hope this
knowledgable company can solve:

Yesterday, I picked up a vintage instrument amplifier at my local boys
scouts sale: A Rock Amp Petros I combo amp.
The amp is in fairly decent condition, considering its age (late 1970's is
my guess) and the price of 15$. The toolex in the bottom needs to be glued,
and some metal corners are missing.

I have seen another Rock Amp Petros I many years ago, back in high school,
and I have also seen several mentioned on the net. However, this one is
radically different. The others were equipped with 10" speakers and had
several controls for distortion, tone shaping and reverb. 
(Like this:
http://www.guitar-museum.com/guitar-46612-VINTAGE-ROCK-AMPLIFIER-COMPANY-PET
ROS-I-AMPLIFIER)

My Petros I, however, only has a volume control (another volume pot on the
back for external amplification). Nothing else. And it is slightly bigger,
equipped with a 12" driver in a bass reflex cab.

Does anybody know of this amp (or of Rock Amps in general)? I only know it
was made in Pomona California.

It looks like this (sorry for the lousy mobile shot, my SLR is on a trip,
must now):
http://www.rumourcontrol.dk/rockamp2.jpg
http://www.rumourcontrol.dk/rockamp1.jpg

The back plate reads 150 watts power consumption. It seems the 10" version
of the amp had a 5 watts amp. But not even a pure class a amp would be that
ridiculously inefficient?

Cheers
John