Another thing I noticed is the difference in quality of the word LEGO imprinted on the stud surfaces.
Referring to below picture (parts from the same Kwik-E-Mart set), you can clearly see the word on the 2x3 plates are much sharper compared to the 4x4 plates which looked washed out. It looks like the latter is probably using a mold that is near the end of its lifespan.
Also, I can visually see a difference in the shade of grey between the 2 actual parts, though you might not be able to tell them apart from this image.
TLG's QC is dropping while at the same time their product pricings kept on increasing.
Just saw the instructions on the net so decided to fix one. For those of you too young to know, it's a cassette tape. Back then I used it to store computer software for the Spectrum and Amstrad computers.