This cover bears a 10cBRCL1A. Although this stamp is not extremely rare (as is the case for the elusive 2pCL1B of 1943) it is rare enough used when compared to the other printings of the 10cBR, most being extremely common, and is extremely rare on cover in 1939. This cover was mailed to Germany.
Here is the 10cBRCL1A
...and here is a detail of the date, March 22, 1939. According to Merlo, there is a first day cover in Moscatelli's collection dated Mar 16, 1939.