I have reattached the drawband on my Erika model 8 which I think is very similar in design. Both German machines.  I know on the Underwood number five the band has like a little hook at each end right?

On my Erika it was a more traditional setup where you have to make a knot at one end of the band and then manually wind up the mainspring and feed the knot through the metal cut-out. As you wind the mainspring, you can see the spring retracting away from the outer edge which gives you enough room to be able to get the knot in through that little notch.

This was actually one of the easier drawband re-attachments I had done because everything was so accessible underneath and ideally positioned.  Compared to other machines were almost impossible  to do it by yourself.

But I'm not familiar with the triumph perfekt per se, so I'm not sure if this will apply. I will say on most machines the hardest part is actually feeding the drawband, or fishing line or whatever, from the point where it attaches on the carriage back to the main spring.  I went to Home Depot and bought the smallest diameter metal rod they had, and then I asked them to cut it and about the 18" Mark.  I taped one end of the string to the end of the metal rod and fed it through that way and it was extremely easy (it's almost impossible turning the typewriter on its side and trying to just dangle it perfectly down through that very narrow space)

Great advice--thank you! Yes, that's what it looks like. I will try this repair. That was very helpful: thanks for taking the time to write such a detailed response.