Bolt Head Markings Chart
Is it metric or imperial? low or high tensile? what grade of material? It can be a minefield trying to understand all the different types and variations, where do you start? If you want to match what you have, this chart is a good starting point. Once identified, matching the thread type and pitch is going to be a lot easier.
Metric
Imperial
Refer to the Imperial vs Metric Strength Chart for strength comparisons.