The profile of the grip frame on the Six Series double actions was changed about the time the first 100,000 guns were produced. Therefore guns with the 150- prefix usually have the old grip frame and prefix 151- and later have the high back grip frame. Cut over was not 'clean' at a certain ser. no.
The low back grip frame exposes more of the base of the hammer and has less of a curve to it where the web of one's hand would be on the grip back strap.
Compare the grip frame on this gun with the grip frame on most of the other guns listed and I think you will see the difference. There is a 6" blued low back also listed but they are more common. Stainless production started about half way or 3/4 of the way through the first 100K guns so the stainless low backs are a bit "rare'.
Hope this helps,
Terry T