I have an F3c and B6sb. I bought the F3 new and I thought it came assembled. The gem was used when I got it and it was assembled. The F3 has a pittman can motor. It is a decent runner in my opinion. The B6 had an open frame motor. I replaced the motor because the armature connections seemed to be hitting the brushes causing a constant clicking noise. It is a good runner I think.
I have heard that in the 50s-70s or so, the tariff structure we had made it much more expensive to import a completely assembly locomotive. Max Grey and USHobbies and others imported "kits" that were really just a few screws to put together the sub-assemblies. The USH ones I got new in the box (old new) also required you to install the motor with screws and solder 2 wires. The loco for example came with the chassis completely assembled but with out lead or trailing trucks as I remember. The superstructure was totally assembled except in many cases for the ashpans and the weights. So they met the requirement to be a kit but were really very easy to put together. In my opinion, don't let the word kit scare you off.
You simply screwed the weight into the boiler. Screwed the motor to mounts if it was separate. Then turned the boiler over in a cradle. solder the wires to the motor. Then fit the mechanism down into the boiler. One large screw goes up through the cylinder saddle at the front, and 2 screws go through the back of the cab to attach the structue to the frame. then use the included screws to mount the lead truck and traiiing if any. Tender assembly was similar without the motor or weight.
It may be that some came through already wired too. I think they liked to ship with the weight and the motor packed separately to avoid damage.