There has long been a hole in the rear cover of the RR501. It's under the label and allows tuning the RF section (I think). It's in an inherently safer location (normally against the wall) and the tunable IF transformer beneath it pretty much blocks access to the circuit board itself so I believe UL was OK with it - IIRC the UL logo is on the rear label of the RR501. However, this hole in the front cover would concern me as it might allow contact with line voltage which is present at the rotary switch contacts. And it's at a height that's particularly accessible to inquisitive toddlers. I would return it or deep-six it if I had toddlers about. Or, you might epoxy a piece of plastic over the hole - that would be safer than a sticker although it will probably score a low SAF.
UL has a website but you can spend an eternity there trying to research something like this.
And, I would hazard a guess that, if they have a large inventory of cases, they have an equally large inventory of labels with the UL logo so IMNSHO, the lack of the UL logo is telling.