"a good signal" // "once the (firing) mission has begun" // "no longer address each other by name, and instead simply use keywords as the recipient / originator are obvious" (ie it's one person transmitting to a lot of receivers)

Almost, "a good sig" is a good signaler - the operator of the radio. It takes a bit of skill and when you get one on each end that can get into a good rhythm it's fantastic.

Yes, the fire mission (FM) will be opened by the FO (Forward Observer), and then they'll drop the callsigns - so something like this:

2 this is 21, fire mission battery, over. 21 this is 2, fire mission battery, out. # from here they will go to abbreviated callsigns, so just stating theirs (it has been a while since I've done the FO's sequence of orders so it may be out a bit :)). 21, grid 12345678, range 1000, bearing 3759, over. # and now they drop them completely so 2 will respond with the readback: grid 12345678, range 1000, bearing 3759, out.