French base, Nam 1954
I sent this pic below to the Royal School of Artillery (Larkhill,England) asking if they knew what the holes are for, and this was their reply--
Thank you for recent enquiry regarding the holes depicted in the Vietnam gun position. Whilst I am unable to give an official or authoritive answer, I have canvassed opinion amongst a number of personnel within the school and the general consensus is as that suggested by the RA Museum, viz personal protection for gun crew from incoming fire. It is believed the holes would have been mechanically drilled hence the regular size.