The original microprocessor was called the 8051 even though about all you could buy was an 8031 partnumber (8031 had no internal program space).

The 8052 later became one of the enhancements and now 8051 and 8052 are used interchangeably. The only use pattern I've detected amongst the choice is whether that individual prefers odd or even numbers. :)

-Jay C. Box

PS: I prefer 8051 since its more historically pure.