According to EveryMac.com, it sounds like the 550c uses a different module than the 520 or 540.
http://www.everymac.com/systems/apple/powerbook/specs/mac_powerbook550c.html
http://www.everymac.com/systems/apple/powerbook/specs/mac_powerbook550c.html
Question is, does this model number for the PCMCIA ( M2995LL/A ) matches the ones for the 520/540 or is it different?