As the production of #71's went along, there were improvements to the design. Later models have quite a few differences, especially in the toolbar mounting hardware. The biggest difference between the 70 and 71 is the drive chain tensioning adjustment. 70's used three bolts through slotted holes to adjust the packer wheel axle to tighten/loosen the chain, where the 71 used a spring loaded idler sprocket with-in the chain housing. The down pressure tensioning adjustment was improved and made more reliable on the 71's. 71's also offered improved, more supple suspension on the "flex" frame.
The first 70's used roll pins to attach seed drop drive sprockets to the shaft they run on. The last of the 70's and then ALL the 71's employed a threaded axle and sprockets.
71's had provisions to attach a fertilizer system, however few were actually sold with those fert. units.
I've got 2 of each. (70's and 71's) If I was buying, for my money, there isn't enough difference in the 2 to warrant a significantly higher price for the 71's. The "time saved" in adjusting chain tension while changing sprockets is about a minute. You won't need a wrench to adjust the chain on a 71 where you will on the 70, but you DO need tools to change the sprockets themselves, so no real advantage there. The down pressure adjustment on the 70's is a little crude in appearance compared to the 71's, but they BOTH work adaquately and very consistant. The only time you'll see any advantage to the suspension differences is IF you plant in a rough seedbed at VERY high ground speeds (for a planter.....above 6 to 7 MPH)
A NEGATIVE with the 71's (comapred to a 70) is the 71's DO NOT like to have the packer wheel turned in reverse. They have a tendency to derail the chains if you accidentally back up with the packer wheel still in contact with the ground. (And that does happen) You have no way of knowing if the chain derails until your corn doesn't come up) 70's tend to ignore accidental reversing.
Long story short, I own both, have USED both, and can't see ANY difference in their ability to accurately plant.
One cause of "double seeding" is often too large of a cell on the seed plates and/or inconsistant sizing of the seed itself. With the move to modern PLATELESS planters that will plant different sized seed at accurate spacings, I've noticed less effort by seed corn producers to consistantly size their seed.
