I don't actually use nylocs due to their habit of bringing the stud with them. plain nuts lightly pinching the rubber have never come loose on me.
My brother in law has a metric frame on a '73 750 and one tank that I lent him wouldn't clear. I can't remember now which tank it was. I suspect that if the frame is a bit oversize and the tunnel a bit under then it can become a squeeze.
You have a good memory L.A.B. The black tank with the repair though was just one that I had lying around and used as an example (wouldn't want you to think that I collect Commando tanks !)
My "problem tank" is actually this white one. I have had it back to metal and it has no repairs and had no filler under the original white / red / blue (could've done with it though !)
Apologies for the poor photo angle. I was a bit worried that the helper was going to drop it (it was full at the time) and wanted to get it over with quickly. The oil underneath resulted from the top of the tacho cable leaking incident.
On the underside, you can see a mark where one of my twin pull throttle cables which ran over the top tube had caught and rubbed when fitted with four or five rubbers at the front (I now have six) and a rubber above the rear bush as well. I have never had any of these problems with the "seamed" Roadster tanks.