Complex sentence: The major response would be the 1937 introduction of the brass threepence coin twelve-sided piece had been introduced in 1937 since threepence worth of pennies or halfpennies was heavy and inconvenient, and the silver threepenny bit was deemed too small.
Split into simpler sentences: The major response would be the 1937 debut of the brass threepence coin. This twelve-sided piece was introduced since threepence worth of pennies or halfpennies was heavy and inconvenient, and the silver threepenny bit was deemed too small.