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Interesting. Perhaps they did it because it was cheaper to make?No so. Around that time the well known and respected Vivitar branded 70/210 F4 constant aperture was on sale. I have one and it is quite a good lens on my Minoltas. as were the 70/210 lenses by Minolta, Canon and the SP version of the Tamron that was probably the best of the bunch. All were either F4 or F3.5 constant aperture.
The Vivitar was not made by them there were several versions made by Komine, Cosina, and Tokina. You can tell who made the lens by looking at and comparing the serial number. Mine is a Cosina made optic. Some were better than others, By repute the Cosina versions were the best of the bunch. I don't remember the website that has the details but start by googling the name of the lens and expand on that.