THere were also 'travelling' inspectors who would go between the factories in a certain area and batch inspect components. So you'd get a, say S4 over on-its-front or back E who'd be putting his mark on many parts from all over the south midlands.
This was certainly the case with the Ministry of Supply examiners who inspected Sten gun parts. One, based at Reading regularly travelled on a train from Bristol to Bath stopping off at factories along the way - Swindon, Theale, Hungerford right up to but not including London. Inspecting, examining, trouble shooting and stamping all the way