I used to have the K88 and had to plug my own filmbacks too.
With filmback mounted, jiggle it... is there some play? If so, it may help to use the thinnest felt you can find and press it into the recess on the body end (buy the type from art shop in black which you can cut to size from a sheet). Thiness is important, or else it will be hard to mount the back later.
It is possible its from the darkslide slot too. If you remove the screws from the face plate of the filmback, you can get at the slot and replace the felt if its worn. Another trick I learnt from an old master (A mr Wong of Wong&Munn)which he used on his hassies is to glue a flap on the body so that when the darkslide is out the flap covers the slot. You may want to try this and see if the problem goes away... if it does it means your problem is at the slot. You can change the seals in the filmback or else leave the flap there.
I also notice your frame spacing is off. Re-read your instructions for loading... the OT backs have a particular loading sequece that MUST be followed or you will have over-lapping frames.
If memory serves me (I sold my kit 3 years ago), you line up the arrows of the film and the red ones in the film insert. Then load the insert into the filmback, open the rear window... wind until you see "1" (or something else, read instructions) ... then close window , mount back and **** shutter and begin shooting. Failure to do the rear window step will lead to off spacing.
