Yes, this is a practical problem with '51/'52. The roots of this discrepancy are as deep as in Intel... They introduced a "feature" line, 8xC51FA/FB, which was in fact '52 compatible; and then the "extra RAM" line, 8xC51RA/RB/RC. The AT89C51RD2 is direct descendant of the latter, through Philips' 'Rx+ and then 'Rx2.

It appears, that the manufacturers perceive "8051" as the generic term, and "8052" only as one of the many derivatives. Note, for example, SiLabs' nomenclature.

JW

PS. The AT89C51RD2 IS downwards pin compatible; however, code might NOT run straight off - see ISP working at reset (check if it does or does not run when external memory is selected - I don't know), fuses (might prevent external code execution), internal XRAM (which by default is ON hence prevents from reaching external XRAM), x2 setting (hence external bus timing)...