Ok, so... we're talking about specimens for bend testing. Let's keep in mind that some materials don't bend very well, especially after welding, and, some folks are only working with sheet metal so don't need plate thickness PQRs.

"For base metal thickness less than 3/8 in, the thickness of the specimen may be the thickness of the base metal, "
     Table 4.3 tells us that when PQR testing materials 1/16" - 3/8" thick, production thickness limitations are 1/16" thick minimum to 2T, and when PQR testing on material less than 1/8" thick the production thickness limitations are 1/2T to 2T (T= thickness of material PQR tested).

"except that M-23 and M-35 materials shall be a maximum of 1/8 in thick"
     I'm not sure about this, but I think it's almost impossible to get these aluminum and aluminum bronze materials to survive a post CJP weld bend test if over 1/8" thick. Hopefully someone else can provide better insight.

"(excluding alloys C95200 and C95400 for which 3/8 in is required)"
     Again, not sure of the reason behind this, must be something to do with these particular grades of castings, but yes, I take it as 3/8"T required.

"Figure B.2A states deviation in base metal thickness welded with F-23 filler metals only. Is alloys C95200 and C95400 only to be qualified on 3/8 in specimens, not allowing thinner test specimens? This is how I am reading this but doesn't add up to me."
     Perhaps these alloys should not be welded with F-23 filler? Not really sure...

"and I am assuming the last sentence ("For metals less than 1/8 in thick, the specimen thickness shall be the thickness of the base metal.") is referring to M-23 and M-35 materials correct?"
     I take this as considering all materials except for C95200 and C95400.

No doubt this section is poorly written and in need of some serious word-smithing.
Perhaps we'll get some more comments from folks more familiar with these materials and we can send suggestions for improvement to the B2.1 committee.

Tim