(Illegal) alteration of a receiver (drilling that 3rd HK hole -or- removing the blocking bar from an RR Uzi) wasn't the point of my post...there's zero ambiguity there (or least there shouldn't be).
When one "paints" a clip-on RR HK as having a "goofy status," they better be willing to acknowledge all the other transferable machine guns in good standing which fall under that same umbrella...the RR Uzi with slotted bolt, RR FNC, RR 10/22 for example, off the top of my head. The HK and Uzi examples are not worth less because of their status, IMO...it's because of less flexibility of the their configuration. At one time, the RR clip-on HKs actually sold for more money than sear-gun HKs.
There's plenty of BATF history and documentation which runs counter to Sterling Nixon's famous clarification letter of the early 2000's. Yet there's still those who believe that BATF will be "rounding up" all those "improper" HK's and Uzi's any day now...and have been saying same for the last 20 years. I own no clip-on RR HKs, RR Uzi with slotted bolt, RR FNC or RR 10/22...but I would not hesitate to purchase for the right deal.
Sterling Nixon in his short tenure as Chief of Tech Branch wreaked all kinds of havoc with his stupidity...the clarification letter relating to the above, arbitrarily switching the registered plate on Maxim guns from left to right and of course the "shoestring is a machine gun" letter, just to name a few. He was an idiot and created confusion for the ages...