I think most of us have had this type of boss and plate it's just the older type is more common and without images the first type will come to mind first.
Yep I've had a lot like that ,there is a bit of a knack to fitting the blade plate , you just hold the plate up with one hand and turn the plate so the drive side of the boss lines up with the plate and tighten the bolt with your other hand , I'd suggest using a rattle gun to tighten the crank bolt if it's not tight enough it will come loose.
Once you line up the plate with the boss you can mark the boss and plate or use a centre punch ,that way next time the plate is off you just line up the marks.
I have used the old boss with that blade plate , the old boss being round with no drive tangs , if you swap the boss you also swap the washer to the old cross washer I put up before.
I've also used this boss with the two drive tangs and cut the tangs off to use a plate that doesn't have the drive tang position on the plate.
