A discussion with David Gesswein about PDP-8 models: July 2012

David worked with two PDP-8's during June-July 2012: his Educomputer 8, and the MARCH
club PDP-8/E. He also looked over my PDP-8/F. These descriptions are part of the lables, 
on the front panel of the machines. We had a discussion about how these lables
by DEC, relate to the hardware inside the box. - Herb Johnson

David: My PDP-8 is an 8/M which is the same as your machine. They called it
an 8/M when sold to an OEM and an 8/F when sold to an end customer.

> Herb: Are you saying my 8 is an "M" also? The front says "8/F".  What
> distinguishes the F versus M, beyond a designation? I"m aware the E
> uses lamps and the F uses LED's.

D: By same I am saying the hardware is the same. DEC labeled it an
8/M when it went to an OEM who would resell it, and 8/F when it
was directly sold to an end user. Nothing I know of distinguishes an F
from M other than the label.

8/M's may be more likely to have third party boards since the OEM may use
them to save money or they were special cards for the particular use.
Special board for machine control were common for OEM's to add. Mine
had the ETOS board and third semiconductor memory which I assume was added
by Educomp? when they were configuring it to be sold for school use. They
also sold it with the ETOS operating system and other software.

> H: OK, I was a little confused by what you meant by "the same" versus "how 
> it's labeled". See if this description is correct:
> 
> Your educomputer 8 (8/M) and my 8/F are physically alike to boards and 
> power supply. The MARCH club 8/E is somewhat different because it has 
> slightly older boards, and two backplanes. The 8/E front panel is (I 
> believe) lamps versus LED's as in your system and mine. DEC labeled 
> their 8's (in this series) as /M for OEM's to modify for resale, as 
> yours was; 8/F (or 8/E?) for direct sales, as apparently mine (MARCH's?) 
> was/were. Other than these considerations, these machines are very similar.

D: That wording is ok.The MARCH 8/E is lamps and my 8/M and your 8/F are LED.
 In case you don't know which differences are due to the
model and which due to age:

The two backplanes is only an option with the 8/E. The board configuration
difference is due to the age of the machines not the model. The 8/E, 8/M 
and 8/F were all discontinued in 1978 and for machines sold at the same
time would have had the same boards. 

[That said,] I'm not positive if they shipped the
8/E with bulb front panels only or switched to LED later. The LED board
will work fine in a 8/E. The bulb board won't work in an 8/M or /F since 
the supply voltage for the bulbs isn't present.

From what I know, the 8/E's weren't labeled differently between OEM and direct sales
They did have some ordering options on front panels. You 
can see some half way down Web page linked below. I don't really know what the
designators were for those options.

http://www.timefracture.org/PDP8.html 

-David Gesswein