Hi, Bob... your stamp is a Scott # 406, or a single from 406a...
more likely the latter...
There are a few positions where watermarks are very difficult
to detect... but, whether you are successfully able to locate
a watermark (or not), it's a 406, as there does not exist any
possibility that it's any other stamp...
To learn more about watermarks, and also access a formidable
database on W/F's... visit the link below...
http://www.stampsmarter.com/1847usa/index.htmRandall