HP-21S modified?

[jebem]

Hi,

I was curious about the PCB variants on some of my Pioneer specimens.
Here it are:

HP-20S 3404S:




HP-21S 2939A:





HP-32SII 3525S:




HP-42S 3219S:
What about that strange looking object underneath the back cover and over the pcb metal plate?
It looks like a 4-pin connector to me.
I never noticed it there, but as I never opened a 42s I really can't tell.
Anyone?