In this case the Main of the title is the Gas Main. This book was first printed in 1929 to encourage householders to cook with gas, this edition dates from April 1941. The temperature of the oven settings was indicated by letter and not number. Not everyone could read well which is why the simpler system of Gas Mark / Regulo 1, 2, 3, 4 etc was introduced. This edition was published during the time of the Belfast Blitz, nevertheless it was printed on a good quality of paper.