Okay, I'll try bending the front edge slightly. And I will take a photo of the side of the reel when I remove the next roll from the tank.
I am now spooling with the non-taped end first rather than the taped one. As to the latter, I rather like keeping the tape on, as it's not the end doing the damage.
BTW, I was wrong about 6x6 negs being unmarked; the flaw appears there, too, I find.
Good advice from both of you re steel reels. I have spent plenty of time practising the friction method, for want of a clip I could understand the function of (see photos). As I said, the film I subsequently ruined is what I use now to play with. I may try again at some point, but at present I see such reels not as a solution but as a source of further problems. I really am determined to continue to use plastic, even at the cost of a flawed frame on every roll. You have all been very helpful in narrowing down the possible causes to this one. I just want to find a fix.
So on the roll to come:
1) I'll bend the non-taped edge before feeding it onto the reel (AP still or perhaps even a fresh unused Paterson);
2) I'll leave tape on or peel it off (if I peel it, I'll advance the roll a little father along the reel -- not sure if this will make any difference, though).