I appear to have a Italian PO Levant General Issue 10c Orange Buff variant SG 4c (all corners not altered), which even in 1969 was catalogued at £350. I wonder if there are clues that can determine if this is likely to be genuine. I think the overprint is under the postmark, which appears to be a "6" although it could be a "9" . On the back, someone has written "30" which refers to the SG number for the stamp issued for Italy, rather than the PO abroad. This is from a regular collection of Italian states, it was tucked in with other "Estero" overprints.
I have two things to ask -
Are there other things I should be looking for to determine whether this is likely to be genuine?
What is the current catalogue on this variant, and what would be a reasonable expectation for a quick sale?