Just a bit of update from a related post, which has a link to the Andover Norton valve specs, which include the length specifications. The current Andover specs are as follows:
Commando inlet - 102.3 mm (4.028")
Commando exhaust - 101.9 mm (4.012")
Atlas inlet - 100.2 mm (3.945")
Atlas exhaust - 99.5 mm (3.917")
The Andover table also lists overall lengths for earlier twins, as well as specifying the various valve diameters.
Note that these measurements are not the same as the ones I measured on old stock Commando valves. Andover lists their lengths as from the tip of the stem to the "bottom of the tulip", and I measured the lengths from the tip of the stem to the highest point on the bottom of the valve. Might be that Andover means something different by "bottom of the tulip". If you look at the valves, you will notice that the bottom of the valve has a rounded contour, and extends below the edge of the seating surface on the intake, and below the valve margin on the exhaust. I suspect that would account for the differences. Specifying the length as Andover does is more accurate in terms of the functional length of the valve, but it is more difficult to measure than the overall length.