Hi Robert,
The original masts on Baker built Seals were made by Proctor which has become part of Selden.Those on later boats built by Parker were supplied by Z Spars as advised by Marvin.
The comprehensive manual for the 26/27 advises that the mast was a type Z 270 (136 x 86mm) at 10.135M (33'3'') long [7/8 rig with top taper] and the boom a Z160.In fact the Z270 is no longer produced in Europe,the nearest being the Z 265 ( actually 143 x96mm).
The web pages for Z Diffusion (the French arm of Z Spars) give a great deal more data on the section properties.
Your local mast makers will no doubt be able to match the section.
It's worth contacting the Regalia/Manuals (under 'About the PSSA') secretary Di Drew to buy the excellent technical manual for the 26/27.It has lots of basic data and tips concerning the rigging,sails,other stuff.
Peter
P.S. Geoff - I will volunteer to scan the manual and provide a pdf to Di Drew if it is useful for this kind of distant enquiry.