No worries Tomo.
When I did it all I spoke to Paul Roman (Roman Autotek) who are the importers for all things Momo, I think they started in the 80's from memory.  You can order from them over the phone as their website doesn't list the boss. Their Ph. 03-93304443.
That is the 48 spline.  The box has description HUB K5613, the receipt has HUB 5613 D, but if you order it from them (the safest way I found) he will know exactly as he was going to update his records after I fitted mine.  You will need to grind a few mm off the rim of the shroud which sits under the boss for clearance, if you've taken your wheel off you will know what I mean, its an easy job. 
Also with this boss the indicator cancelling does work, apparently some bosses it does not.
The horn button I got from 'Killer' (Chris) send him a PM if your'e a member or this is his website 
CSK Landrovers
Edit.  I just checked his website he still sells them, 15 pounds.
No I didn't hook up the button I don't think you can, I find it just as easy to use the fitted horn on the stalk.  I actually used a bit of silicone on it and glued it to the back of the dress ring as the SAAS wheel dress ring hole is slightly smaller than a Momo.
Mick
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