What is the difference between a T/O (Test Only) and a VR Stamp?

A T/O Certificate, which stands for “Testing Only” is obtained by an organization for the purpose of in-service testing and performing minor adjustments only; this process can often be done in-line. Alternatively, an organization that holds a VR Stamp will be certified and audited for quality assurance of their complete repair process including disassembly, internal inspection of components, machining, assembly and testing of the Pressure Relief Valve.