I take it back - I THINK IT MIGHT WORK.
It looks like the sub-fender support on some V7III models utilize the same front fender support bolts as our MKI and then ALSO uses a second set of bolts just behind them - but what is interesting is it looks like the hole is there in the frame already on the MKI.
It's a little hard to see because my HB Junior racks are partially covering it. But sure looks like it's in the right place and if you're swapping fenders and that mount you should be good (assuming not HB Junior racks).
A couple of pics.
My MKI (you can just see the hole I'm talking about peeking out from behind the rack):
Here's the V7III from the same angle:
And here's the V7III peering under the fender mount/seat latch brace assembly from the front showing both bolts. On the III models this subframe actually sits inboard of the end of the main frame tubes):
So double-check that your frame looks the same (it should) and it is or can be clear (when you remove the MKI fender), but I THINK you're good to go.
PS - NOTE in the photos that I've got HB topcase racks on both of our V7s.