Yea, but for the 1m round Primestar, I think the stock horn is lookin' for a round dish, just like any common LNB.
Stand in front of the dish, remove the LNB, and look at the dish through the LNB holder hole (if it's got one), or over the mount screws.
Your view of the dish, from that perspective, should be that the dish is round.
I ran around for the longest time thinking every Primestar dish, if it was to be retrofitted with a new LNB, must keep the stock Primestar-supplied feed horn.
That's only true with the oblong dishes (and you can see by looking through the LNB holder that they are).
So, now I'd love to find one of those big round Primestars like the OP has.
Hey, a lizard's gotta dream! - :up
And instead of worrying about the high price of a QPF-031, I'd probably equip it witih a QPH-031 for half the price.
QPF-031 = flanged; use your own horn
QPH-031 = has horn built-in