I'll have to look up when the change was made. So, later.
But it was before he "/M" generation.
It was indeed more than cosmetic.
The answer to "why" should be obvious.
There you are, with your EL, winding gets slower, and finally it stops, mid cycle.
No other charged battery available.
And you can't charge the thing that is in the camera until the camera has completed the cycle.
Which it of course will not since the battery is flat, right?
So what are you going to do?
You could get a D-cell type battery, put that in the camera, and use the cover (which will not fit over a D-cell) to short the battery and fuse. The camera will complete the cycle then, and you can charge the EL-battery.
But what if no D-cell?
Would it not be great if all you needed to do is connect the charger and fill the battery up again?
But you couldn't. So they changed that.
However, the cover, including the lever, will fit. The change is in what is behind that thingy.
Palpas was introduced in the 3rd generation. That means the 553 ELX was the first in the EL-line that has Palpas. No EL or EL/M, or even 500 ELX has Palpas.
It is not an add-on, not something that can be sprayed on quickly: the old interior covers without this coating would need to be changed for newer ones with coating. Possibly bits of the interior chassis too. The mirror pan as well. And the rear shutter doors.
And those new bits may not fit the older bodies, needing perhaps even more bits changed.
The 'upgrade' would not be worth the no doubt substantial amount such a rebuild would cost in parts and labour. And after all, the new coating is fine, but only a micro-improvement.