Desaxe engine?

Atlas are slightly desaxe in order to fit the bores within the barrel bolt pattern.  I understand that offsetting has very little effect if it is small.

Measuring piston drop is actually quite accurate enough for setting ignition on long stroke undersquare engines, especially if they have larger cylinder sizes, but it requires either vertical plugs or near enough vertical to get a narrow probe in close to vertical.  Or some slightly elaborate trigonometry calculation, which would be even more complicated with a desaxe engine.  So it's easy on a 16H but not so easy with an ES2.  There's also the issue of identifying a precise point on the strangely shaped cylinder heads to measure against.

But at the end of the day, the best point is still found by trial and error from the book figure, so it's more important to have an accurately repeatable method than to worry about which method.  That's why it's not such a good idea with under-square, relatively small cylinders like the Norton Light Twins, where timing angle will be more sensitive to errors in piston drop measurements.

I time both my SV and OHC Singles by piston drop (to avoid stripping the primary) but the Dommie by degree wheel on the crank.

The Atlas and all Bendies are like that, I believe, as it was the only way that they could get the oversize barrel to fit into the existing crankcase mouth, rather than for reasons of good engineering.