I read many times that 486SXs were rebranded 486DXs with a defective co-processor unit, don't really know if it was rumors or just plain fiction.

Seems perfectly plausible. CPU binning has been being done for years. Made an 8-core chip, but one core is faulty? Disable some cores and sell it as a 6- or 4-core chip. Chip won't run at the frequency expected? Sell it as a lower-spec CPU.

https://en.wikipedia.org/wiki/Product_binning#Semiconductor_...

There is some discussion here:

http://www.os2museum.com/wp/lies-damn-lies-and-wikipedia/

My guess is that if there’s any truth to it that was early on before the process was fine-tuned. It’s hard to believe Intel would have tolerated a process producing that many errors since they could use the same die space to make more smaller chips.

probably

intel used to sell cheap core solos back in the core duo days, which were just duos where one of the cores was faulty so they disabled it.

Same with the F model (no iGPU) chips today. People have done teardowns and found that more often than not its there on the die, just disabled because its faulty.