You Guys are all forgetting that lore wise that Chineese people from San Fransico (Fo2) are already able to produce T51b Power Armor. Heck You can even get a quest so that they get to examine and figure out how to perform hardening proess on T51b Power Armor. Further more back when Enlave troopers roamed around San Fran by using the Exon Valdez tanker as a means to travel from the Oil Rig, the San Fran inhabitants learned of APA. If You carefully read info from all Shi palace computers, You get to know that they are planning to produce APA's too, just it will get them some time to solve the ceramic compounds issue for APA plates they sttmbled upon. Spoiler: Proof that Shi produce T-51b. You can Purchase T51b PA in both Shi shops in chinatown although spawn rate for PA is not 100% in eigther of them, so if You don't find it go take some time in the desert to kill stuff and come back later. Simply waiting in town will not help.