There are only two types of P3's, the earlier Katmai's and the later Coppermines. See
this.
The 500 MHz Katmai's, which I guess is the kind you want, can be id'd as follows:
MHz..... FSB Stepping.... Form...... Volts CPUid Retail..... OEM.....
500........ 100 kB0........... SECC..... 2.00 0672h N/A...... SL38F....
500........ 100 kB0........... SECC2... 2.00 0672h SL3CD.. SL365....
500........ 100 kC0........... SECC2... 2.00 0673h SL37D... SL35E...