Ok, With W&G's there is really only three categorties
Early - Glass tension mechanism (Rare - worth a bunch)
Automatic - Non-adjustable automatic tension (no glass, common as hen's teeth but sell well because of the crazy ornate treadles)
Electric - Could be portable or desk. They sell for about 1/2 of the treadles
of course there were (like all other machines that were popular) licensed copies ans well as knockoffs
And they made them much later than that for the industrial market (up to the 60's)
and I have to add that they are THE most silent machines ever.... no adjustments of any kind on the "automatics" They sew very smoothly with a beautiful locking chainstitch