James:
Is the boiler shell made of wrought iron or steel?
It could be possible that the thin readings that you obtained in your ultrasound testing are due to a lamination in the material. If so, I would be more concerned with that than the lap seam.
On the Maine state government website, it was interesting to read the state's boiler law. It generally references ASME B&PV Code. It is interesting that there is a specific prohibition of lap seam boilers except for minature or under 15 psig.
ASME B&PV Code does allow riveted lap seam boilers provided that the shell is less than 36 inches in diameter and under 100 psig.
Paul