Swift River, Yukon Cancel Missing Date

I have just picked up a cover from Swift River, Yukon with this cancellation



the cover is obviously philatelic but when I got it home I noticed that the date 20 had been added by hand but only to one of the strikes.

The cover may have been addressed by the person who sent it as a souvenir and it is possible that the date was added much later back in England. (A second very similar cover by the same person was from Teslin on the 15th June.)

I just wondered if anyone has any other Swift River cancels with the day slug missing.

AQ