A Small Nicely Tied On Swedish Cover With Tb Label

(Nothing special) I like tied on covers

Actually, did Sweden have automatic letter facing machines in 1955? It is about the time experiments with these started in olther countries. If so, the automatic letter facing module may have misread the anti-tuberculosis seal for the franking, and fed the item incorrectly into the canceller. If not, a clerk may have done so. The automatic canceller cancelled the seal and not the stamps that, subsequently, were cancelled by hand. (In Stockholm 80, five days later?)

That thought crossed my mind (but am not smart enough to follow up on) If they did have auto facers would the label have to have some sort of luminescence or just be way more colorful than the stamps?
As you said, five days later. One station E, one G (both Stockholm 80). Maybe on the way to the Air Post bag the other clerk caught it? Interesting to think about. Thanks

Automatic letter facing machines were experimented with in the thirties. I doubt these older models used luminescence or electric currents. The GPO in the UK started experimenting with such machines in 1957. If and how it was done before that time, I do not know. But I think the facer machines, already, existed.