Chassis ground bolt size?

Does anyone know what size this bolt is or if theres a specific part number? It's for grounding the battery to chassis. The 1st pic might be what it is that I also seen grounded but not much to go off besides the "10.3" marked on it.

 

Likely an M8x1.25. That's probably a 10.9 marking for Grade 8 equivalent. If you're going to use that hole as a ground, I'd run a thread chaser to clean out the rust first. Once it's clean, use a random bolt from the hardware store or the autoparts store. Take off the bolt in red and bring it in to the auto parts store to double check that M8x1.25 is what you need. Home Depot/ Tractor Supply usually have thread checkers in store.