Out of curiosity I looked at the Tech Manual [TM 11-826] for the E thru I versions. All it says about the spark gap is "In the H and I models, filter choke L4 has a closely spaced spark gap across its terminals." There are no further references to it except in the same paragraph explaining why it's there. According to the specs for those models, L4 is rated at 10KV. For the earlier models [A thru D and some of the E's] it was rated at 5 KV. So much for the Tech Manual answer, which I presume agrees with yours. If I could remember what the breakdown voltage of air was, I could compute the spacing roughly. It's unusual that the military then did not make a comment of maintaining the spark gap; they did everything else.
Must be like the R-390 s-meters Top Secret!! The height of spark gap technology. In the end it wasn't my problem , got the rig up and going.But I certainly learned something.