Offsets Needed For Simplex Prox Cards S12906

Those are 125 kHz cards using a 36 bit S12906 format, which is proprietary to Simplex but does not limit supply sources.

A sizeable number of non-Simplex/Tyco suppliers sell them.

What readers is this system using?  Generally if the reader is non-proprietary, it can communicate via Wiegand or OSDP to the controller regardless of format.  (C-Cure is among these systems).

Appears the cards make use of a custom Wiegand format.  Additionally the cards are probably based upon HID's 125-kHz proximity protocol, though they could also be based upon a MIFARE or I-Class 13.56-MHz contactless smartcard protocol, as well as several other possibilities.

Building here, if the readers are indeed non-proprietary, then they can probably support access cards making use of a different - perhaps more open -  Wiegand format, though operating on the same protocols, 125-kHz proximity or 13.56-MHz contactless smartcard.

An example of perhaps the most open format is the industry standard 26-bit Wiegand format.  Cards with this format are widely available. Of course, with more openness also comes greater vulnerabilites.  Here occurrences of duplicated codes are known.

However, if Software House's C-Cure can handle multiple Wiegand formats, then moving the customer in question to a more widely available format may certainly be an option worth exploring.