See UG-37(a), definition of "F" factor. Then see definition of "integral reinforcement" given in UW-16(c)(1).
Generally, a separate loose "reinforcing pad plate" does not meet the requirements of "integral reinforcement". Thus you cannot use the F factor to gain benefit of reduced reinforcing area through cross-sections not taken through a longitudinal "cut". But may not be a big deal.
Generally, you would use integral forgings, a special forging that has the nozzle neck (and possibly the flange too) integrally forged with a heavier barrel to provide additional reinforcing area when separate plates are not desired. This may be for fabrication cost savings or other reasons (high temperature process, etc).