I wouldn't say the stamp is meaningless, but apparently the fabricator (once granted the stamp and paying the annual fee) is somewhat self-policing (until caught). I believe there is supposed to be a field audit every three years.

I suspect problems are more likely with fired pressure vessels (power boilers - "S" stamp) and especially the small niche of locomotive boilers, than with ordinary pressure vessels (air tanks, etc). Your tanks are relatively simple, with no stayed or braced surfaces, thermal stresses, etc.