[BarryGMC]

Well I did a little more searching on my 65. on the drivers side frame rail right behind the cab mount. # 2443634 -3-09 - 57-69 Passenger side same area # 2443639 -1-20 - 57-69 On the front frame horns the 57 is stamped. So the first #'s would seem to be frame rail part #'s the second numbers I believe to be forming or stamping dates for each rail.I am making this assumption based on the fact my truck was the 7th truck assembled on April 2nd 1965 at the Freemont California plant.And all of the date #'s I have found are from January through march of 65. the last numbers I cant hazard a guess what they mean. Now since my truck is a 1500 series it would make sense they are not the same as yours. But there seems to be a sequential pattern here. Here are some copies of a GMC sales brochure that show an artists rendering of a GMC truck chassis, and the frame measurements. The Frame line lists the dimensions of the frame channel under the cab. Hope this helps. Barry