IMR data was with Reminton 5 1/2 primers and a 6 inch barrel.
With the difference in fps I would think this test barrel might have been a "Solid" and not vented,as was the custom back in those days and maybe one heck of a crimp,also !!

We all know that a company might "Fudge" a few 100fps or so for sales but going from a magnum load down to a "Lite" mag or heavy target load , with my 6 inch 686 "Vented" barrel was some what of a surprise.

Its all good ...testing out different powder is fun and I enjoy doing so,just that I hate it when you step into an elevator and say "UP" and it goes....."Down".