IAF Jags have gone through several avionic and weapons upgrades. When originally selected in the late '70s, Jag came with the notorious NAVWASS system, renowned for constant toppling.
The Indians didn't like it and opted for the Digital Attack Ranging Inertial Nav (DARIN) upgrade, which used a French Ulliss-80 IN. DARIN II I think introduced an RLG IN, and DARIN III cockpit enhancements and an autopilot (perhaps the 55 referred to). This latest mod looks to be some enhancement to the rest of the fleet.
The Jag remained in production until fairly recently - I think India received about 20 dedicated strike two-seaters in about 2005-07, and about the same number of single-seaters as attrition replacements.