I cannot tell you if there is a weight difference. It is possible some of the castings could have had their mass reduced. Don't give up on your 1958. It can be restored unless there is serious problems with it. Below are before and after pictures of an early 1954 Jointer I restored. The first shows the Infeed and Base/Outfeed Tables already cleaned up but they were in worse shape than the Fence. I wish I had take more pictures of it before and after.
Mark V completely upgraded to Mark 7
Mark V 520
All SPT's & 2 Power Stations
Model 10ER S/N R64000 first one I restored on bench w/ metal ends & retractable casters. Has Speed Changer, Model 4E Jointer, Jig Saw with lamp, a complete set of original accessories & much more.
Model 10E S/N 1077 oldest one I have restored. On bench w/ metal ends & retractable casters.