I did the speed reduction kit on the 41, easy to do. Seemed that the chain cover was exactly the same to me. However I did use the new one.
The 41 speed reduction kit just comprises of a replacement chain cover, fresh gearbox sprocket, and a larger roller sprocket and longer chain to suite. If you look inside the back of the original cover you'll see a lump just inside which has been removed in the new cover that comes with the kit. This is to stop the larger sprocket from touching that internal lump.Speeds:
I realise that many Pro users complained about the Harriers being too slow, but they appear to be all over the place with the new Harriers. The 48 Pro without the reduction kit travels at around 3.4 mph, fitting the kit brings it down to 3 mph.
The Toro Prostripe does a speed of 3.2 mph in top gear, so the Harrier with speed reduction is only 0.2 mph slower, and you're stuck with that speed full time.
The variable speed system that nearly all the makers have adopted now, Stihl / Viking, Cobra, and Hayter to name a few, seems very inconsistent. Speed will change (increase) as the belt wares, so some identical machines will be faster than others. Cables are an absolute nightmare to change on these systems as well.
Although I'm not big fan of the drive changes, the manufacturers are in a position where they can't please everyone all of the time. That I can fully understand.