I looked through my DirecTV Phase III dishes out in the garage.
The first and only three sets had the cast-in horns.

Then, I spied one more dish. Low 'n behold, it had the hardware you described! - :up
After removal and examination, I finally understood some of the comments you made above:
The only new parts I needed were 4 4mm x16mm bolts per LNB and some silicon sealer.
I dissasembled the DTV triple and removed the feedhorns. Beautiful Horns.
But they have a blade in them that needs to be ground off.
Also the mounting flange of the new feedhorn needs to be ground down were the bolts go by.
I hope to test these against some other LNBs soon.
Will look forward to your comparison against the 105° stock horns, if you have any.
I guess those aren't nearly so desirable as I once thought. - :up
I went out and used the afternoon sun to take a picture of the goodies.
On the left, the Phase III body, with the horns above it.
Lower left, a stock feed for an elliptical dish, I suspect for the -
other- manufacturer of the 121° SuperDish.
Then a soup-can LNB with Primestar feedhorn for an elliptical dish.
The weird rectangular feed is off a very wide elliptical DirecWay dish.
(it's threaded, but I think by drilling, it'll bolt-up to a Dish FSS LNB, if desired)
And finally on the right, the LNB and feed off a 105° SuperDish. (it twists and detents for linear or circular operation)
I'll do the grinding and make up the mounting ring/plate when I need to use one, but just having these parts on-hand gives me a warm feeling.
Thanks so much for eagle-eying the removable feedhorns! -
