So I am wondering at what year they started making them without strips. Here is my November 1982 note (which the catalogue says should have a security strip):
It seems that the two issues without the security strip are variants of issues that had the strips, namely 1978-Jun-01 and 1982-Nov-04. I'm not sure about 1983-Apr-07 though, as I don't have an example and the Numista catalogue comment had listed it as being without the strip before I became referee for Costa Rica.
It appears that your note would be P#238s2 and I've reached out to a system administrator to have the year shown.
Cheers,
Titus
Numista referee for banknotes from Canada, USA, Costa Rica, China, Macau, Singapore, & Taiwan.
Costa Rica • 20 Colones (Series C)