Welcome to the forum Mike! There are several non-precancels in the collection. ( Why does my spell checker not recognize "precancel"? ). The two strips of four aren't, the Eisenhower pair isn't, the 10 cent with the two flags and the Martha Washington next to it aren't and the cut-out 1 cent with "Paid" in big letters is not! Also, the pink 2 cent and the one next to it on the third picture are regular cancels! I hope you do not consider this negative feedback - just trying to help out!
What I think ikeypikey is referring to as a precanelled envelope (bottom image - Tampa?) is third class printed matter bulk mailing. PL&R stands for Postal Laws and Regulations. The minimum per-piece rate for all third class bulk mail was one cent from July 1 1928 until July 1 1952. As of October 1 1932 each piece was required to be labeled 'Sec 562, PL&R'. On February 25 1949 this was changed to 'Sec 34.66, PL&R'. Other codes were used later are the minimum rate had gone up.
The Tampa, Fl. cut square is a precancelled envelope. "Sec. 435 1/2 P.L&R." (authorized in 1928) refers to the section of Postal Laws & Regulations which authorized usage of precancelled envelops, who used them, and was a part of the precancel. Tampa, Fl. was authorized to use precancelled envelops on June 30, 1930.
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