Hi Chris,
I don't think these bolts could be termed 'military specific'. At one time I worked on a lot of equipment (not military) with UNF fasteners. I think it was 7/16" UNF bolts that had 5/8" A/F heads, but nuts were 11/16" A/F, then you would come across same bolts with 11/16" A/F heads. Going to Metric, some 10 mm bolts have 11mm A/F heads rather than the normal 10mm.
The bolts you mention on the RB44 chassis, could have smaller heads due to restrictions in getting a socket or spanner on them maybe.
cheers Richard