I can not for the life of me distinguish between SL8 and SL9 in Ted Proud's book. They also have the same prewar years of usage between 1936 and 1941. They are not that uncommon and is an interesting paquebot cds but after the war, for whatever reasons, use reverted to the straight line types, either letters alone as in the very rare SL10 with the funny looking "Q" or letters within parallel lines or a box.
This does not look very interesting being an obvious CTO on the first day of use. However, we have a postwar cypher stamp with a cancellation date of 2 DEC 1947. SL8 was brought into use again postwar between 2.12.47 and 7.3.48. I have been reliably informed that postwar covers of this type are quite uncommon.
Having found a commercial cover with a postwar cypher stamp with a date possibly of 14 FEB 1948, I am obliged to downgrade it to relatively uncommon rather than rare.