Brian,
check out my tables of frame weights, including SB310 in 60cm size :
http://www.ece.ubc.ca/
I think you have over-estimated the weight of your fork, which is at most 810 grams. If you compare your frame to my 1977 Raleigh Comp, 22.5", which is roughly the size of your frame, your frame is 3.75 = 1702 grams, which is almost 15% lighter! I think you are a lucky winner !! That would be a very, very early 753 frame, if indeed that's what it is ... I'm assuming you have a sloping-crown fork, and those forks weigh a TON (but are major cool, imho ...)
- Don Gillies
San Diego, CA