Second number on frame?

These numbers are quite common on later WD16H frames. They're stamped in such a place that they clearly weren't intended to be seen with the fuel tank fitted.

We have no factory ledgers for this period but it is fairly safe to assume that when the Ministry of Supply / War Office insisted on 'matching' engine and frame numbers, Norton were not in a position, especially bearing in mind wartime demands and production levels, to ensure that pre-stamped engines and frames were married up on the production line.

The implication therefore is that the matching 'engine' and 'frame' numbers were stamped at the end of the line before delivery. However, the factory would still have required a serial number for quality control and internal use. Many WD16H engines have a number stamped on the timing chest prefixed 'WO' and many WD frames have a number stamped on the top lug.

My best guess is that these were frame and engine build numbers. They had no significance once the machine had been completed (but are interesting all the same) and if we had enough, might just help us to assess if the visible frame numbers were in the correct area and perhaps expose any re-stamped numbers.

I've started to do this with the 'WO' engine stampings which are more visible but am unlikely to ever have enough detail about these hidden frame numbers to draw any conclusions.