KZ1000 cam bolt hardness

1981 KZ1000J engine, I was installing the cam caps back in and discovered three of the 16 were stripped. I have some 15mm helicoil arriving today, but some of the bolt threads are also mangled and should be replaced. Are these bolts hardened? Some of them have the number "7" stamped on the head, but that's not a normal hardness rating for metric bolts. I can find replacements at Ace Hardware in standard, 8.8, and 10.9, so what would be suitable? And would it be ok to just replace all 16 bolts with new ones, or should I mix them and try to keep as many originals as possible? (The bike will never be "factory original", so that's not a concern.)

Japanese hardware as used on bikes on the KZ era often have 4/7/9 marks indicating the strength

Metric 8.8 / SAE grade 5 / Japanese bolt marked 7
Metric 10.9 / SAE grade 8 / Japanese bolt marked 9

I'd use the 10.9's for that application. One issue: hardware store metric hardware often uses oddball hex head sizes (like 11 & 15 mm). Not sure about you, but I hate that.

Any idea why the bolts marked as '7' would have been in there then? I guess it looks like everyone agrees on the 10.9 bolts though, so I'll see what I can find this weekend. Thanks.