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I just checked mine. 5 x 18mm countersunk head and it does fit the Speedo drive as well.
I tried to repair the original front caliper on my 400/4 but it was beyond help with both the fluid union and the bleed nipple snapped in the housing. I bought a pattern caliper from DSS along with a master cylinder repair kit. The master cylinder was an easy rebuild. The hardest part is removing the internal circlip to enable piston removal. You need to buy a cheap pair of long nose or circlip pliers and maybe grind the ends down to fit. Easy job to do though and you keep the original master cylinder with the correct exterior finish, the pattern ones are painted black.
Try some heat on the offending parts. It may be enough to break the joint. Need to be careful with the heat on the aluminium caliper body of course.If that fails then soak the body in a container of diesel oil or penetrating oil for a few weeks.Good luck!
Use one of the disposable cartride plumbers torches and warm the alloy round the fittings till slightly uncomfortable to touch then use a long six sided socket with a tee bar so you are turning evenly not twisting. Never failed to get one out yet. If the pipe union is really bad cut the pipe off flush with the top of the nut and follow same instructions.
I woild say send it to me and i would do it for you but two trips accross the pond would get a bit pricey!!
If it does all gp wrong the caliper is the same on the 500/550 four and the 500 twin so plenty out there.