1963 is a weird year. GMC has hydraulic clutch. Chevy has mechanical clutch except trucks C50 and above. Lozzyuk states it is the combo style master. "dual". The brake master cylinders vary from 1 1/8 and 1 1/4 depending on the axle. 1 1/8 Master seems to be the norm.
Chevy calls it an integral master. A normal definition for dual is for brakes only and is a safety redundant feature.

Lozzy, you need to check the slave cylinder also. Both the master and the slave are available in US as you can see. The slave cylinder is Rockauto: Dorman CS36124

You can use these part numbers from me and 3B to buy them anywhere you wish. They are not Rockauto specific part numbers. You can also cross reference them here or in UK. Overseas shipping is always a killer. There is a rebuild kit also. The rebuild kit may be much cheaper due to having to ship a core to be rebuilt. There are no cores left at the rebuilders for this older master it seems.

Rockauto is a drop shipper. They do not have product on shelves. For multiple parts, It's possible each vendor would have to ship to UK separately.

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