Again from the Birmingham Proofhouse: 3 tons = 8,943 psi (approximate conversion to CIP transducer measurement). 4 tons = 11,913 psi. That's about the same difference that the Birmingham Proofhouse gives as the current spread between service pressure of an 850 bar gun (10,730 psi) and proof pressure of said gun (13,920 psi). Looks to me like somewhere along the line, service pressure and proof pressure got mixed up.