Tourer rear seatbelts dilema

[KSM62]

I installed a leather interior in my Mk1 tourer and stored the original cloth seats in my loft for the next 12 years. I am now reinstating the original interior and prepping the car for sale. The middle rear seatbelt on both sets was working perfectly until I swapped the seats over. I now have the problem of the belt not freely releasing when the rear seat is down on both sets, cloth and leather. The leather seat needed minor repairs so I stripped it down and inspected the mechanism. Everything appeared to be in working order including the cable between the latch and the inertia reel. I'm having difficulty figuring out how the system operates. There is a small ball and lever mechanism within the belt reel which I had assumed released the belt when it was not upright. However, it seems to work in the opposite way. When the seat is lowered the mechanism stops the belt from unwinding. If you've had this problem before you first discover it when the seat can't be lifted back up and you have to struggle under the seat to unbolt the belt from the floor. I was lucky that the seat squab was not in place then. My dilemma is that I can remove this part of the mechanism and it appears to have no detrimental effect on the seat belt operation, but it may be an important safety part. This will eventually become an MOT problem. I'm sure this is not an uncommon problem.