Well, I FINALLY received the FAA documents for N4526H/15-304 from the FAA. It shows it was manufactured as a stock PA-15 on May 13, 1948, and was licensed "Experimental" on August 17, 1948, "Certificated for the purpose of Testing preparatory to type certification;..." with a Continental C90-8F engine installed. The airworthiness application includes a statement that obviously includes a typographical error, "This airplane is similar to the PA-11 except for power plant installation."

The file includes a letter from Piper Aircraft Corporation stating "Will you please cancel the complete file on NX 4526H, Serial 15-304 as this aircraft was completely destroyed by fire at the Cub Haven Airport, Lock Haven, March 3rd, 1949?" The file contains no references to a model number PA-18, or a serial number starting in 18-.