Honda-SOHC

This thread looks like it has what you want: http://www.sohc.co.uk/index.php?topic=6955.0

He's probably buggered up the front of a perfectly good front mudguard fitting that mud flap.

I can't see any extra holes or fittings so suspect it may be fitted using the stay mounting screws.
What beats me is why anyone would dismantle the brace from the guard in the first place, before reassembling it wrongly.
Can't see whether it has been riveted back together or the rivets replaced by screws.

My thinking/belief is that if tyre size is wider then the swinging arm is wider to allow for this and the frame tubes at the top are correspondingly wider to keep the shock mounts the same distance apart at top and bottom, but I may be wrong in this assumption.
I know when I was looking for a rack that would fit that some I looked at were too wide at the shock mounting points, being probably intended for larger bikes. Fortunately someone on here helped me out with one that was intended for the bike.
It's not easy to measure accurately in situ but the distance across the mounting points on the 400 is approx. 170mm.

Part numbers will only tell you so much.

Quite agree.

But in this case I think it is true that the 400 has things like tyre sizes in common with smaller models such as the 250/360, whereas 500 and up have larger section tyres.
Therefore the frame width at the rear and swinging arm are larger to accommodate the wider tyres, and the mudguard is correspondingly wider.
I don't know by how much, and a 400 guard could possibly be fitted, but I suspect it would need spacers at the frame mounting points

This listing seems to suggest that a 750 guard will fit; personally I have no idea!
http://www.cb-four.com/cgi-bin/shop/__Rear-fender--Honda-CB750Four-K0-6-CB500F-CB550F-new-original__370000349184-01f_04.04.html

It just seems very odd that the diagram and user guide would refer to this second position when it doesn't physically exist on the switch.

I think the position does exist on the switch but the wording may be a bit confusing.
This is the wording from the 400 handbook - it doesn't have the buzzer but I'm pretty sure the switch works the same way.

[ Guests cannot view attachments ]

I decided to get one of these tools, but where from?
DS list it but aren't expecting stock until 8th April so I consulted Mr Google.
That came up with a company called Kramp (http://www.kramp.com/shop-gb/en) who supply agricultural equipment and have branches/agents nationwide.
I got in touch with a local one and now have the part, cost £4.68 inc VAT (DS price £3.90 inc VAT but plus delivery, when they have it).
So if anyone else needs one that could be an option to try.