which book is this?
I'm not really familiar with "MA9"s, and i think you misread something.
Most of the MU cars that are of the modern varieties are all MA-1(insert letter here).
For example, The MA-1A is the Arrow I (1968), the MA-1B are Silverliner IIs (1963), the MA-1Cs are Silverliner IIIs (1968), MA-1Ds are Pioneer III cars(1958), the MA-1Es are Single Silverliner IVs(1974), MA-1Fs are Married Pair Silverliner IVs(1975), MA-1Gs are Arrow IIs(1975), MA-1Fs are Single Arrow IIIs(1975), and the MA-1H are married pair Arrow IIIs(1976-79).
the MP54s will all retired by the end of 1980, and by that time, it was just a handful of them on assignments like the Princeton Dinky.