We haven't worked on a supercritical boiler but we have done a number of high pressure boilers using P5B material for main steam that have used threaded and seal welded plugs.
I've never seen the kind of problem described by the contractor.
I don't have the standard, but for $20 you can get one here:
http://www.pfi-institute.org/Product.htm
Not much help.
Charles Hall
Response to your questions:
1) Yes, we have seen the seal welds crack on threaded RT plugs on supercritical units and some drum units. Typically, they have grossly oversized welds and fail due to creep cracking in the HAZ on the P22 pipe side. I have seen a few welded with E309 electrodes (for some unknown reason) that cracked due to thermal stress.
2) We have also used full-penetration welded caps. You can get them from Gibson Products in TX.
3) I don't have it here at home, but it shows the thread details, hole sizes, and plug details. "Representative" refers to a threaded plug and means the dimensions may be different than shown. But, it still has to be properly designed for the pressure since the threads carry the load.