If there is "questionable" engagement between head of fastener and wrench spanner or socket, try this trick: Look at tool, see how there is a rounded edge before there is positive engegment of tool to fastener? Grind that shit flat! Keep it cool so as not to lose hardness. Even highest quality tools have that "roll", I have 1 of every size that way, even Snap On's which I use in critically normally overtorqued (such as valve adjustment nuts) fasteners. Try it, you will think its new again as your tool is as engauged as much as it can be.

+1

My DT360 is the same and I raised the query here as well!

Whilst what MarkT has said will be correct, who knows what the original nuts might have been replaced with over time and it is these that you have to deal with now. I reckon that there were at least 2 different sized heads on the equivalent nuts on my DT360.................!!

Borrowed a couple of sockets, 12mm and 13mm, 12 went easily to the top of the nut but not sit on the nut, too small, 13mm was too thick to fit the access hole, then remembered that I have a small socket set that has 13 as it's biggest. Dropped that down on an extension and fitted ok but the wrench is to "weak" to crack the nuts. Tried a 12mm from same set to confirm that it's not 12mm. Going to look for something else.

We never know what previous owners did, it can't hurt to see try SAE sockets for fitment too, since you're really just looking for the most snug fitting six sided socket.