Here is a 1931 Tonga air mail cover with a 'mailed from Tin Can Island' cachet on what looks like a tape strip.

Here is a detail of the cachet:

The cover is by Roessler. I'm pretty sure this isn't traditional Tin Can mail, as the postmark is Nukualofa. Not to mention, it's air mail.
It is from 1931, which was the 'shark' year that forced the shift from swimmer mail to canoe mail, but that is almost certainly just a coincidence.
Any thoughts on this? (The main Tin Can Mail website does not show this.)
[edit: Also, the rate is 2d, which seems just a bit low.]