Not sure about a "stamp" perse', but the AISC will issue a certification of various degrees to a fabricator. And the AWS recognizes the AISC's yearly audits and will issue a cert for AWS Certified Structural Steel Fabricator.
John Wright

Curtis
"Stamping" for structural steel fabrication is not always required. Some job contracts do not require any qualification other than a number of years of experience in the business. However most large contracts or customers will require the fabricator be certified to the appropriate level with AISC. A fabricator may request exemption from the requirement on an individual basis but that doesn't always work.

If the contractor contacts AISC or goes to www.aisc.org , he will find information that will prove valuable to him. The folks at AISC and their auditing company QMC, are very helpful and the info is free of charge. Even if the contractor doesn't want to certify at this time, he can at least be taking steps toward that goal, which can only help him to be more competitive.

As John, mentioned, the AWS has a Certified Fabricator program that should also provide some benefit. However I have no personal knowledge of that program to share with you. I understand it should be advantageous for smaller fabricators to go that route but as an AISC fabricator, we didn't have a need for the additional certification.

Chet Guilford