I saw one today on an auction site, they said it was a 3 screw. Is that correct? S24xxxx serial number. Blue with 4 inch tube.Can't find anysuch animal in the book.
Actual model marked 20s are very rare. I would check with the seller to confirm model mark and number of screws as I would think it should be a 4 screw model..
A serial number in the S240XXX range would be a -2 model variation, but could easily be marked MOD-20 (three screw frame with an extractor rod with a left-hand thread). This model was not very popular in the late 50s and 60s and you might find unusual markings as well as a mix of old and new parts.