As I said- I do not think I have ever come across a Stihl mount bar that was not either 0.050 or 0.063 gauge- your bars measure worn 0.063 gauge and if you can still find/read the bar etching- probably say 0.063 or 1.6mm on then near the stud slots.
You can run your old chains if the drive links are still in good condition I guess- hasn't killed you yet has it?
My guess is some dubious dealer at some stage either had an excess of strange to Americans (very common here) 0.058 gauge chain he was passing off to the unsuspecting, or just didnt know any better.
Running thinner gauge chain can cause more debris to enter the groove and wear nose sprockets- bar groove, can twist and bond in the cut as the drive links are less supported, carry less oil in the bar groove- but you have gotten away with it thus far.
Several worn out chains, couple of worn out bars- I would be seriously looking at the clutch bell drive sprocket as well- if it is well worn it will not increase the life of chains nor bars.