For what it is worth: the cap you are talking about is PN A 000 868 00 30 and it was used in 108, 109, 111, 113, 114 and 115. So it is on the parts list for Pagoda.
It is on the parts list for 113044 only.
But then I see for example a seat belt A 110 860 20 85 with this cap shown as part of the kit, used in some of the 113043 cars as well (chassis 002982-003839 and 004647).
The same seat belt was for 113044 up to chassis 000655.
For the rest of 113044, the seat belt kit was A 114 860 16 85, where drawing suggests it also contained this cap.
For the cap itself, the note says: 113044 as of chassis 000656.
Both are for US, SA 55 686.
Wrapping up: this cap most likely (not 100% sure) was part of the above mentioned seat belts for 043 and 044, but it is specifically mentioned to be fitted in 044 as of chassis 656.
I would have thought - it was meant to be there, but... my wild guess would be: it came with the seat belts when seat belts were ordered. Installation was another story - the part may not be easy to install in a Pagoda, it does not fit there well and might not have been installed or might have been removed by owners.
The other two sets of 3 point seat belts from those days, both for outside of US and Canada, I think:
A 115 860 17 85 (SA 55 639) was not for w113 (bottom anchor clamps 90deg). It does not have a cap.
A 115 860 08 85 and A 115 860 09 85 (SA 55 639) there is no information as for the model of the car they went into. There is a cap on the drawing.