Took the 2nd cylinder into a workshop near work this morning. They called me a little later and said they had another option for me.
They could get me a brand new aftermarket unit for $75 more than rebuilding the 2nd hand unit. The rebuild was going to be $225 and the new unit is $300. A new unit from Mazda was $568.

The aftermaket unit is made in Japan by Seiken Chemical Industry co.,ltd.
The part numbers on the box are;
SM-Z572
Mazda - N001-43-400B
IBS JB1819
The new part looks identical to the Mazda part.
Just thought I would share this in case someone here needed to replace their brake master cylinder and wanted another option.
J