Rick,
"5 Diamond" refers to the type of cut checkering on the forearm. From
'52-Mid '60's remington produced 760's with several forearm variations
(tootsie roll, plain-no checkering, basic cut checkering pattern and the 5 Diamond cut checkering). The "5 Diamond" being just that, 5 diamonds cut into the forend of a high grade walnut. Remington never refered to it as "5 Diamond", collectors nicknamed it the 5 Diamond. Different grades of the 760 received different forends(ADL, BDL, CDL) Guns of that period that had the "5 Diamond" forends were of a higher grade and were produced in limited numbers. In the 760 collector circles, a mint condition 5 Diamond 760 is a highly sought after rifle.
Another note, Remington rarely stamped the grade of the rifle on the receiver. If you run across a 760 with "BDL or "CDL" stamped on the reciver, you have found a real keeper!! Hope this helps.