It would if you bolted the reel to the shoulder mount on the B pillar. If you install a 254 correctly it’s a three-point mount if you don’t it isn’t.
I think the confusion is the difference between a static belt and an inertial reel. I think a three point static belt has three mounts, an inertia reel four. Because the reel is mounted down on the floor and it needs an anchor point at shoulder height. The reel wont reel if it's upside down or mounted at an angle hence the need to mount it at floor level. In essence two of the mounts on a three point inertia reel bolt to the same place, making it three "points" but four holes. A static belt has three separate points, shoulder, hip and hip (one of which will be the buckle). No doubt there are specialist belts the will reel upside down or at an angle for racing/rallying etc but in a normal car your not going to bolt 1/2 a kilo of steel 50 mm from your noggin. Is this what you mean @davidoft?
Have a look at the ‘3 point automatic belt’ diagrams 4, 5 and 6. http://www.securon.co.uk/fitting/extras/254fitting.htm Obviously, you wouldn’t fit a 254 to the front of a Bay, although it would work if you did, but it would work perfectly safely at the rear as 3-point mount inertia reel belt.
Just throwing this into the mix, aren’t the rear belts using the over the shoulder brackets not 4 point belts? Belt reel, shoulder mount, arch fixing, buckle! Just been through this making some recessed mounts for a converted panel van.
I tried the 254 in the back with reel mounted on the upper bolt and it was rubbish, mainly as couldn't fold out bed, so I bought Davidofs brackets and a rivnut tool as no bottom bolt hole and used some front belts and its brill.
I wouldn’t use rivnuts for all of the mounts, but just to locate the bottom of the bracket where the top (important) mount is a factory captive nut I think is fine. Mot tester chap was happy with this setup.
This^ but make sure the top bolt is good and tight. If the bracket pivots on the bolt in an impact the shear stress is on the bottom bolt (and rivnut) and if it’s smaller than 7/16” (about 11.1mm) it’s not a seat belt bolt. 7/16” UNF is the standard thread size for all seatbelts.