Prior to WW1 production of  indigenous magnetos was small and the majority of engine makers used Bosch.  During WW1 many new Companies were formed and their initial efforts are almost always near direct copies of Bosch models

With early production Petters bought in many small components and may have done the same with magnetos. A variety of magnetos appear including CAV, and perhaps the Simms SF1 and Bosch ZE1. The latter can bear serials which match the engines for date but they were, and are, a common retrofit since they share key dimensions with other magnetos which were definitely original equipment.

In perhaps late 1914 or early 1915 Petters switched to the Thomson-Bennett type AC1Z (whose designation and design suggest Bosch origins).

In 1919 or 1920 the model changed to the Thomson-Bennett type AC1. This shows only minor differences to the earlier AC1Z

During 1922 or 1923 Thomson-Bennet were absorbed by  BTH and shortly afterwards Petters began to use the BTH type T-B1. Although this closely resembles the later types MC1 and AF the designation is suggestive of Thomson-Bennet design origins.

The BTH T-B1 was supplanted during late 1923 or early 1924 by the type M1 and later by the AF2 and AF4. The AF4 remained the standard  through to the end of production.

Before 1932 a cast magneto gear cover was fitted. from 1932 on a pressed version was used.

Up to 1931 the magneto was mounted on a separate bracket and driven by a crankshaft skew gear. From 1932 on  the drive-side main bearing housing incorporates brackets to support the Calibrater lubricator and the magneto which are both driven by the crankshaft skew gear through a common shaft.