SHOW OFF!!
Easy when you have all the equipment free-of-charge at your whim isn't it? I wish I had a lathe, and a milling machine wouldn't hurt either.
That should be exactly what he needs, with two spares to boot, good on you.
The only thing that I would suggest is that when these are being installed, check to make sure that the bolt shank is not longer than the combined thickness of the handle bar and riser. The reason that I suggest this is because it has been reported on this forum that Gen-Mar makes the riser for the police ST1300, and whether it came from Honda or Gen Mar they are one in the same part. If that is the case, the 43mm length bolt specified by Honda should be the correct length. I would check to make sure that the extra 17mm is not causing the bolt shanks to bottom out against the bridge causing them to tighten up before sufficient clamping force of the three parts has been achieved. I have no idea what the distribution of the extra 17mm is, but 17mm is a significant difference in length if a significant amount of it went in to the shank. Good news is that it is an easy fix- just thread the bolt a little more if needed.
Andrew, thanks for all the input and support on my effort to help
@dlim
I recall other posts that noted that the PA spec risers were made by GenMar for Honda. I made my length decision based on that
1Cruiser website that said "the risers ... raise the bars up 32mm". As I said before 18 + 32 = 50mm, but I defer to those with more first hand experience.

(<-- intended to be just one guy bowing in honor, but our emojis don't include that one.)
And my free bolt supply didn't include 50mm ones, only 60mm... so it's easier to shorten a bolt that's too long than to add length to one that's too short!
So, everything you said to David is valid. I see three paths forward for him if they're too long as delivered (which I think they will be).
1. If there's sufficient thread available, simply cut the excess from each bolt, dress the threads at the end and call it good.
2. Same as 1. but if the threads don't go up the shank far enough, cut more with an M8 die as you suggested before cutting.
3. Add ~17mm worth of washers, or better yet a spacer, under the head of the bolt to ensure the clamping that this bolt is there to deliver. This would result in the cover being raised 17mm higher than it would have been otherwise, but that wouldn't be too terrible.
Then again, I could return to the shop and use a parting tool to make one set 43mm long. Yesterday was convenient because it was during our weekly robotics meeting anyway.