[Hallicrafters] HT-37 Question

[Mark K3MSB]

Just finished the recap of my HT-37 tonight.   I've owned it since the late
1980's and it's always performed great,  but I figured I was on borrowed
time with the original caps in it.    I replaced the two HV caps years ago,
and took care of the rest today.

The model label is just to the left of the fuse holder on the rear of the
chassis.  However,  under it is a square slot flanked by two small holes
that looks to be for a slide switch of some kind.    I don't think this was
added by a previous owner as the model label is over it.

Any idea what went in there?

73 Mark K3MSB