Bridgeport Mill head Q's

In Digest #109, Jon Elson said:
<begin quote>

> Well, almost. I had a 1941 M-head machine, SN 1388. I replaced
> the M head with a J, and it was a huge improvement. The great
> availability of R-8 arbors and end mill holders is a big advantage.
>

<end quote>

Can anyone point me to some information concerning the differences between
the M head and the J head? I know essentially nothing about milling
machines, apart from being able to distinguish them from lathes. Which is
the earlier design? Which is more desirable? (I'm guessing J here.) Also,
the same info about round ram vs not-so-round ram.

What part of the machine is considered the ram vs the head? The head is the
part that has the motor and quill, right? The ram is the part that allows
translation motion of the head in the y-axis, right? (moving closer to and
farther from the face of the operator). Are they swappable among Bport mills
of a certain age, i.e. do the bolt patterns that need to match up match up
etc.?

Thanking you in advance, as I attempt to inoculate myself against a
particular strain of hoof-in-mouth disease. Private replies or simple URLs
appreciated, although you are most welcome to reply on-list if you think
such a basic topic is of interest to the general population.

--dan fox