Where was the mark in relation to the spiral - was it on a frame that was on the inside of the reel where it could be touched by the funnel or whatever depending on the type of tank you are using? On the outside where it may have touched the side of the tank or contaminated fingers? Or somewhere in the middle, protected from being touched by the rest of the film? Was it on the edge of the film where it may have picked up contamination left on the reel? Contamination by chemicals on your fingers while loading the film? I seem to recall some kind of clip used for loading two
My best guess is that this was caused by the small plastic rod near the end of the spiral (at least there is on mine. I have no idea what purpose this rod serves, but if your mark is in that location, it would appear to be your culprit. If this is the case then this is a long winded way of saying that Dr Bob got it right the first time...
If this is not the case, the options are endless... Can you show us a scan?
Cheers, Bob.