The Ford 917 mower is an OK flail but is much lighter duty than a Mott/Alamo. It's not uncommon to see dents and even rusting through on a 917. If you want to make it work better and have more power, put Mott knives on it. They will fit on the existing rings and don't require as much HP to run. They are individual and will independently swing backwards and to the side when encountering an object. The one piece Ford knives also don't self clean and will plug up in heavy grass. The reason older flails have different knives is because Mott had a patent on the individual free swing knives. You adjust the rear roller for your height of cut. For just general cutting a 917 should work good.
Of interesting note is that the flail mower was invented by Mott when he was an engineer at International Harvester but IH was not interested in his new mower design so he went out on his own. He tested the self cleaning properties of his "Hammer Knives" in pig manure.
I found an old 907 just rusting away behind my neighbor's house. Looks almost the same as 917. What is the difference in the two?
The knives that are currently on my flail are what I believe to be called scope type. It may not be clear in the picture, but they are a one piece design. They are not two "L" shaped knives on one mount.
Which replacement blades would be better for a nice clean finish cut?
I don't think so. Not based upon the initial photos anyway. That machine was seemingly seldom used, or those are replacement knives and hangers. It's simply not possible from the photo to determine the pedigree of the knives. Given that this machine is somewhere between 29 and 40 years old, original knives would have worn/rounded tips - and the hangers would have grooves worn in them where the knives swing. The knives and rings pictured by the way, look exactly the same as the aftermarket replacements I bought for my 917H. Mine was so worn, I ended up replacing knives, hangers, brackets, and bracket bolt/nut. Got it all from FlailMaster.The mower has the original Ford knives .