a number 10 on a metric bolt is the metric hardness number not thread size. that standard is way different then what most of us know so well including me 42 years useing it as a mechanic on german made garbage. i still have problems. size wise a 8mm is about 5/16 a 10 is between 3/8 and 7/16 11 is about 7/16 12 is another betwwen size ,13 is just bigger then 1/2, 14 is close to 9/16. hardness the higher the number the harder and my brain can't remember just how high is the hardest made right now . we also need to watch the threads as there is a std metric and a iso metric. std is what we and across the pond use while many threads from the far east are iso which is a finer thread then a std metric fine. behond this i'm just to old to have any brains left after all the co in the shops air over the years .
as to my 5er hitch i too let the local reese hitch guy do mine and as he is also the local rhino dealer i had him spray my bed at the same time. when they got done masking it all out and doing the job it looks grand and when asked about removeing and hideing. i got no problem they will never know where it was the holes be be sprayed over after we plug them .