tonearm to fit mysterious pre-cut plinth?

Okay, so it may not be a mystery to you (I hope), but it is to me. I picked up a KM 4001 turntable at an auction. It came without an arm, and I'd like to fit it with one. There's a pre-cut oval in the acrylic plinth, measuring (at maximum center points) 3-3/4" x 1-1/8". Any idea what arm this was cut to accomodate?

I have an MMT arm I was hoping would fit, but it would require major refitting. I don't think the table is anything particularly special, so I don't want to sink a lot of $$$ into an arm for it, but I would like something decent. Any input is welcome, and if anyone owns a KM turntable or is familiar with its intimate workings, I have a couple of tech questions I'd love to share with you... Thanks, Ben