It is important to consider which "options" were considered by the
manufacturer to be user-installable. RS did NOT think that a user
could/should install a serial port!
It's a little like replacing an automobile drainpan gasket - NOT intended
for field replacement.
And, as to the Coco,...
The stories appear to be true that RS DELIBERATELY packaged the Coco in
order to PREVENT expansion, competition with their other models, or even
serious usage. (consider their choice of video mode(s), RF video out ONLY,
etc.)