Many B-Series models can be characterized as prototypes, even those that finally got into production. First off if you have a P500 model then you are out of luck, the architecture is too different to run any of the commercial titles that got released. If not you need to check if maybe you had a pre-production series ROM set. Type in the BASIC commands BANK 15:PEEK(65529) and press the return key. This address in bank 15 holds the ROM code version number. It should have a value of 01 at $aa. Anything else is too old and burning EPROMs with the latest (revision 04) firmware may be all you need.