Where is SRP-Z from? The Stanford license for SRP says: 


  Broader use of the SRP authentication technology, such as variants
  incorporating the use of an explicit server secret (SRP-Z), may
  require a license;


Yet I cannot find a succinct description of the SRP-Z protocol and why it might be useful. As a follow up question assuming it is useful and patented when does the patent expire?
This question both describes the SRP-Z variant and mentions that the patent should expire in three years.
I think the patent in question is this one, but I'm not sure: US 6539479. If it is, it seems to actually have expired earlier this year due to non-payment of fees. Looking it up on the USPTO PAIR says:

Status:   Patent Expired Due to NonPayment of Maintenance Fees Under 37 CFR 1.362
Status Date:  04-20-2015

It is still possible it could be revived after such expiry, but IANAL, etc.