Difference Between 4145H and 4145M

Hi,

Talking about drilling tools like drill collars, subs and crossover for the oil and gas industry, I have found differences for the raw material used for the making, some manufacturers use 4145H and other use 4145M, both of them meet the API spec 7, but i have not found the difference between them.

Can Anybody help me to understand differences between those steels?

Regards.