It looks like the Mott model 88 or 96 is pretty close. Flail Master seems to have the most online information. I'll pull a set and then call them. I may end up buying an original IH knife from Ebay. Then there is no question what the dimensions should be. I also noticed that Flail Master has weld on mounting ears/lugs. I need one, so that would be a higher chance of matching the weight. Can't be too much to order one and see (obviously if I measure what I have and send pictures that would help).
The Flail Master 5/16" hole knives look like a good start. Mine has a clevis hanger. I'll have to add that length to these. They don't show an elongated hole to use what mine are. Something to do!
Fine-Cut Blade (MH 386)
(102086/02988196)
Pin Mounted
.087” Thick
1 1/4” Wide
4.7” Tall
5/16” Mtg. hole