Nick is mostly correct...
I would revise the term "drive platform" to say 'baseplate', as the baseplate in later 1981 models and on was used in both capacities. The drive components mounted to the baseplate were largely interchangeable before and after this revision.
This is indeed a low-end 70-series machine, in fact it is the entry level 70. I bought this model new. It is a 1983 model, stock number is 23701. This is considered a five-cycle machine, with three levels, three temps, an off-balance relay, and two speeds. This model was one of Sears' best sellers in belt-drives, along with a fairly similar 8-cycle 70-series and a fancier 8-cycle 80 series.
This particular example is a 1985 or newer unit. The very first 23701 models, along with some 1981 and 1982 machines, as well as one other 1983 70-series (the 23711) started out with plastic lid hinges. These hinges were dodgy things that did nothing for the machine except make them appear cheap, in my opinion anyway. They came out in 1980 and were gone by 1985. Originally they had been used only in very low end models, then by 1982 in some 60-series units, and the two 70-series machines in the first 1983 models.
A 1985 revision to this machine and the 23711 eliminated the plastic hinges and restored the 1950s hinges to these two models. Typically these hinges do not break, though they can be bent. Usually the two screws that hold them to the lid come loose, and/or the plastic hinge bushings break, causing the hinges themselves to become loose and allow the lid to falter, however I have never seen one this bad before. Often when hinges become loose and lids fall so far back, the finish on the top becomes compromised, which is evident in the pics, especially toward the rear of the lid near the control panel.
I would love to get after that funky lid and all the schmutz on the agitator.
Gordon