(HP41) What is this for a model version with such battery contact?

[rprosperi]

That unit has an HP Battery shield installed. It snaps in and out, it is not permanent, but be careful doing so as the contacts are delicate.

HP introduced this rather late and it was intended to prevent alkaline battery spills from damaging the battery contacts and/or spill into the case and damage electronics inside.

More info and pics here: https://www.hpmuseum.org/forum/thread-21067.html