Hi Don,
that is the form of the mooring strop we use, although I'm pretty sure ours come from a standard lifting supplier in Melb.
We use either a single or double concrete mooring block, approx. 1200mmx 1200mm x 500, then with bottom chain, then strop to surface level.
Having a cat, my rope bridle then goes on, and the whole strop ends up remaining underwater due to bridle length needed. Mono's have either a topchain or rope.
Biggest advantage of using strop is the lifespan: our oldest strop is now older than 8 years, and has been destructive tested ok. You avoid having a middle chain which takes a lot of the wear. Plus the weight difference is much better on you back than all chain.
If you do have a topchain, check where it goes over the bowroller!