Hi,
I have a Topical Encyclopaedia of the USA printed around 1947 and there is no Northumberland Co. in Maryland.
There is a Nothumberland Co. in Virginia (with a population in 1940 of 10,463).
Almost all references (on the internet) put Northumberland Co. in Va. but there are a few (recent and 17th century) that refer to this county being in Maryland.
Cordreys Beach, Northumberland County, Maryland ??