I would have imagined that it kept the KwK 75 as you say. In my imaginary world, the mods to the earliest IDF panthers would have centred around fixing the mechanical deficiencies of the original. Probably a new engine and updated drive train, including strengthened final drive mounting (which you can see as welded gussets under the glacis) and strengthened swing arms and road wheels (which you can't see). The initial mods to the Shermans centred around it's biggest deficiency, poor firepower.
The second round of mods would be what you see here, an even further up-powered engine (the AVDS-1790) of the M60, the IR suppression system from the M60 plus the French 105. What you can't see here is the new tranny to go with it. The bustle is added partially to balance the 105 and partially to locate the radio up into the turret allowing the assistant driver to be eliminated and replaced with ammo stowage for the larger 105mm rounds.
It's at this stage that the up-armouring with homogeneous plate would have happened to reduce the instance of the armour shattering if overmatched. The new AVDS-1790 engine and tranny being more than capable of handling the extra 4 tons of armour and also providing a little extra speed and much more bottom end torque for quick acceleration and manoeuvering. The economy of the diesel was also much appreciated increasing the Panther's range even with the other benefits.
Operationally, during the 1973 war they served with reserve units and constituted the bulk of the second line blocking forces on the Jordanian front until shifted North to take part in the attacks into Syria. While technically seriously outmatched by Syrian T-55s and T-62s, the M51'5 potent 105mm gun and superior Israeli training and tactics meant that the Panthers actually came out well ahead in those tank on tank engagements of the last part of the war.
By 1975 or so the M51 Panthers were getting long in the tooth and the writing was clearly on the wall, but with 20+ years of service in the IDF there were a lot of spares for the remaining vehicles and they found ready homes with the Lebanese Christian forces and also in South America with the Argentinians and Peruvians operating them into the 90s.
Today the last few ex-IDF M51 Panthers serve in Peru as training tanks.
How's that for a comprehensive history that never was?
I think I'm going to paint and mark it as one of the reserve brigade vehicles during the assault into Syria near the end of the war.
Paul