Nearest I can measure with a micrometer and magnifying lens is a bit more - 21.33mm or 0.84". You are right - it is very odd! I was expecting it to be a round number in inches, or on the 2.54mm grid of the connectors themselves. The gap between the connector shells is about 15.5mm = 0.61", but that is not particularly useful for laying out a PCB.
Right, I saw the part numbers, but I need the spacing from one part to the other.
I just looked at this again and noticed that 21.33mm does not equal 0.84". Did you mean 0.54"?
from the drill files provided with the Module it is 740mil or 18.796mm between the headers.
920mil - 180mil = 740mil (inner pins)
If your measure from left to left side it is of course 840mil or 21.336mm.
I stand by my sources! 21.33mm = 0.83976378".
This of course is the distance between similar pin-centres on each connector, so the pins in the left column of J1 to those in the left column of J2 (or the right column in each case).