I have a number of these shafts from a few implements (mostly front mowers and snowblowers and brooms). They are all different brands and models, too. JD, Ford/New Holland, etc. Some Some are spline to spline and others are spline to 1" round shaft, either drilled for a shear bolt or a keyway deal.
I have found a few from U-Pullit type junkyards, and some from auctions where they have a mid or front mower only, and even from Craigslist and FB marketplace.
Just count the splines. 15 as I recall on my F-935 mower and 1070 tractor.
You won't believe what they cost new. Cheapest one I've seen (New Holland cm274) is about $700 and the Deere is over $1000. so, buying a rusty mower deck with a drive shaft for $50 is a darn good deal. Air compressor attachment is also a nice occasional use for a mid-pto. Tank or no tank works effectively. Don't forget the safety-blow off valve !