The Info I have is from an italian web site.This is what I can tell you. Regno:IMPOSTA SULL'ENTRATA INDUSTRIA E COMMERCIO
The first 4 stamps of 20L & 50L have a watermark crown.It is either normal or on the right.So check this.Inverted types exist also.
The 20L dates from August 1940 with last usage october 1947.
The 50 L dates from August 1940 with last use April 1946.
From what I gather,your 20Lstamps should have the watermark at the right.
Because the serial Numbers start from A-000001 to H-786275 with this watermark type.(7.8 million printed).
The 50L should have a normal crown type watermark.It's serial Numbers start from A-000001 to T-564850 (19.6 million printed).
As far as the letter sequence for the 20L & 50L I would say they must be from the very early printings.