That is interesting. Looks like you have one of the old (no longer made) quick hitch to fit the 605/737 with combo spline/flat 3 jaw PTO, but it has tapered 3 jaw PTO. I wonder if someone just swapped the PTO "connection" on that quick hitch from flat 3 jaw to the tapered 3 jaw. Seems weird that set it up that way. Anyway, yes, the 737/605 come with the combo spline/flat 3 jaw PTO, prior to the new tapered 3 jaw PTO that came out in 1995. So your 605 is a pre 1995, but probably early 90s model. Since 1995, most of the PTO is now enclosed in the transmission and in oil bath. This is definitely a better setup as everything except the 3 teeth are inside the transmission in oil bath. Grillo still uses this old style. It works fine. I run both. My Grillo G131 has flat 3 jaw PTO. But if I can have my wear components in oil bath instead of grease I would rather do that. Of course putting the transmission back together is more difficult now that everything is inside the transmission on the BCS.
As to taller tires and tilling, yes it does change the angle. Another thing is if user is very tall (over 6' 4"), it might be difficult for them having to lean over (more back aches), especially on Grillo since they only have 4 handlebar height adjustment positions. If user only tills, makes sense to use 5" x 10" maybe. But I do so much with my BCS/Grillo machines that I would never run less than 5" x 12" except in extreme cases.