It is possibly a fictional name of a mine.

From the List of Scots Collieries there are several:

Ayrshire: Coylton : Littlemill district. Shieldmains Nos. 6, 7, 8, 12 and 14
Midlothian: Musselburgh : Dalkeith district. Fordel Mains

from The Scottish Mining website

http://www.scottishmining.co.uk/Indexes/1944listofmines.html

http://www.scottishmining.co.uk/Indexes/ListMines1893.html

there are other lists for 1854,1860 and 1866 as well

However there doesn't appear to be a Curbly Main listed.

My Grandfather lived in Musselburgh not far from Fordel Mains.

CC