What do the numbers and letters on Dawn vices mean?
Numbers – 3, 4, 4½ etc.
This designates the jaw width, in inches or in mm – post-metrication (1970s) a 4″ became a 100, for example. A 4:

A 3:

L or LP
L or LP on a Dawn vices indicates that it’s a light pattern vice. To use the late-60s 4″ vice as an example, the 4″ standard pattern has 60% more mass than the light so it’s a substantial difference.

S or SP
Standard pattern. One of the most common designators. Heavier duty than the light pattern, not as heavy duty as the heavy pattern.

HP
The biggest vices of any given jaw width! HP stands for heavy pattern. These are much more massive – to use the late-60s 6HP as an example, the heavy pattern weighs approx. 75% more. You don’t see as many of these as you do the standard or light pattern.

M
Medium pattern. You do not see many of these. The late 60s catalogues show a 4″M in the quick action vices but no others medium pattern vices. Bit of an odd one, really, and I’ve only seen less than half a dozen. The below is an example of the very old M pattern which pre-dates the SP introduction in approx. 1930:

This is an example of the 1960s quick action M (thanks Daniel!):

Q, QR, QA, O
Quick action. Comes with a half nut instead of a full nut, so it can be dropped down via a spring loaded secondary handle and the slide moved in and out freely. Often combined with the weight designator – so you might have a 6SQA, for example. QR isn’t a Dawn term but you may see it used by owners/sellers. O isn’t one that they use – if you see a 6SO it’s almost certainly a 6SQ.
Here is a 3 ¼” standard pattern quick release:

OS
Offset. Very useful for holding objects vertically.
Often mistaken for a zero – e.g. read as 60 S rather than 6 OS.
Most metric offset vices lack this designator and are just marked with the jaw width – it’s pretty obvious visually that it’s an offset.
Steel
Dawn’s earliest unbreakable line. Instead of being cast iron (or cast semi-steel, as Dawn used to put it) these were straight cast steel. Won’t shatter like cast iron if struck or over-stressed. Replaced by SG. There were both imperial and metric Steel vices.
SG
Spheroidal Graphite cast iron, as opposed to the standard stuff. Also called Super Grade. This replaced cast steel vices as Dawn’s “Unbreakable” top-level vice. Won’t break like cast iron will if hit or over-stressed. Only began post-metrication.
Woodworking vices
This isn’t my area of expertise, but CMQA is Cabinet Makers Quick Action, JWW is Junior Wood Workers and CMPS is Cabinet Maker’s Plain Screw.