Rear Seat belt replacement

Hi,

We have recently purchased a 1996 T4 Autosleeper Trooper and love it. unfortunately the passenger side rear seat belt is failing to lock and requires replacing, the belt is a securon but has no model/part number details on it.
On my vehicle the belt is mounted high up just inside the back door in a horizontal position and there is no addition pillar loop fitted to guide the belt over the shoulder.

I'm considering purchasing a Securon 507/w lap and Diagonal seat belt as it seems to be the only one available that matches the required description. Before I purchase one, have any other forum users replaced their rear nearside seat beltS on a similar aged t4 and are there any alternative items out there.
I searched the pages but can only find information on replacing the drivers side rear belt which is mounted very differently to ours.

thanks in advance.