Excellent point.
There are those of us who like to use aluminum rails, and some rails have lips:
That’s a Z rail, and the drawing is maddeningly missing some of the most important dimensions. But it does appear to be to scale in which case the nut slot is 6.5 mm wide and the aperture in front of it is 4 mm wide. M3 slide nuts are 5.5 mm wide. So the panel holes must be 194 ± 1 mm apart.
The distance from the center of the slot to the inside of the lip is 3.8 mm. With two rails at a slot to slot distance of 194 mm, the lip to lip distance is 201.6 mm. Any panel larger than that won’t fit.
Vector rails have no lip
They use M2.5 slide nuts. I’m not sure what their width is. But the slot aperture is 0.118" = 3.0 mm, so there’s only ±0.25 mm play there, however there’s also ±0.25 mm play in the 3 mm panel mounting hole, so if I’m thinking straight that again leads to 194 ± 1 mm hole distance. With no lip, the rails do not constrain the panel size as long as the hole distance is right.
If I’m thinking straight.