May I suggest you look at pages 39-40 of Richard Friedberg's "Introduction to United States Revenue Stamps" for a more complete explanation. Because of reuse of the 1917 documentary stamps, Internal Revenue had BEP overprint the documentary and stock transfer stamps with "Series 1940" so that they could only be used in 1940 and 1941. At the end of 1941 all remaining unused stamps were to be sent back and traded for Series 1941 or Series 1942 stamps. Since the retention period for documents was 3 years the stamps could not be soaked off and reused. The precedent for this was the tobacco taxpaids beginning in 1932 (on these the first group was Series 102). The tobacco taxpaids of series 102 were only valid in 1932 and 1933.
Now regarding the so-called "sensitive ink" this was an experiment of an indelible ink for which BEP and IRS were to receive feedback. Apparently the feedback felt this special ink was unnecessary because of the rest of the regulations regarding retention of the documents and devaluation of the stamps after two calendar years.
Again, read Richard Friedberg's more detailed explanation quoting the Commissioner of Internal Revenue, Guy T. Helvering.