If you look up through the grill just on the left hand side of centre, you can get a screwdriver in there to pop the mechanism pretty easy. I dont think they had to be real security conscious back when they made em.

most likely it will be a E1 or J15, these seem to be the most common.

the effort involved to make an A series fit in there is considerable.

what condition is it in?

what plans do you have for it?