VT 520 Terminal

Back of a VT 520 Teminal has a 1"X1" (approx.) rectangular area covered by a removable plastic flap. It is just above the keyboard hole(mini 6 pin DIN).

What is it?

Thank you.

---------- Post updated at 02:49 PM ---------- Previous update was at 11:37 AM ----------

I've got the answer. Its for rom catridge.
Problem is solved.

---------- Post updated at 02:53 PM ---------- Previous update was at 02:49 PM ----------

I've got the answer. It's for rom catridge.
Problem is solved.