Archie, the Magnum .328 were X7 alloy shafts made for Bear Archery by Easton. In the 1972 Bear catalog it says; "Magnum arrows in .324 and .328 are available on special order. The 29" .324 arrow weighs 625 grains; the .328 weighs 655 grains. These heavier shafts are recommended for 65 lb bows and up. For Magnum shafts in other than 29" or 31" please allow two to four weeks for delivery.". That is a stout and relatively uncommon shaft.
Their .320 shaft was rated for 70/80 lb bows.
.316 for 60/70 lb
.312 for 50/60 lb
.308 for 40/50 lb
I like the regular Magnums, never seen any heavier than the .320's. I think it's a cool find!
Hope this helps,
Brad