Material: S-2 (Mod #4)

[SMetcalf]

What is the material type S-2 Mod #4?
I know that S-2 is a Shock Steel, but can't find information for what Mod #4 means.

Thank you!

 

[NRP99]

Something related to modification in alloy composition? Hardening specification/method? I am guessing though. Search in ASTM A681 doesn't tell anything.

Perhaps the context of specification and/or application of material use may give you the hint required.

 

[blacksmith37]

It is a shock resistant tool steel which has S-2 and S-4 grades. My guess is that it is a proprietary modification ( which may be a minor change).