Zinc plate 2C callout?

[Treads0322]

We have a handful of steel fasteners in our older drawings with a description of "GRADE 5, 2C". Our internal part numbers correspond to, for example, McMaster 90126A031. I assume that the 2C designation is meant to suggest the zinc-plated finish but I haven't been able to find a standard that it would be associated with. The only references I've found are ASTM A193 (appears to refer to a heat treat for SST) and BS1706, where it doesn't mean much without additional context. Our customer is now asking for clarification so I need to come up with an answer. Can anyone give me a hint? Thanks!
-mT-

 

[BillPSU]

I have found what 2B stands for, material: medium carbon steel treatment: quench and temper.

I could not find anything on the 2c.

I would also admonish your company for not having the standards referenced on your prints available. Someone called out the 2C at some point in time and now you do not have records. Sounds like a record keeping problem.

Bill