Long story long...
Bought my L85A3 from Extremeairsoft back in Nov/Dec 2022 & within a couple of dozen shots the charging handle fell off (a typical fault apparently), when I went to repair it I noticed a burst thread on one of the dissassembly screws so I summised it was a returned pew...
I reckon the cumberbund theory is correct, the loop on the rear panel goes through a hole in each of the shoulder straps & cumberbund (adjusted to fit you of course) with a quick detatch cord securing them.
Been planning this for a while & happy with the outcome, still need to find a switchset... when I find it.
disclaimer... They liked them big in the early 80's, I feel the need to grow a 70's porn tash now tho.