The Will of William Selby was executed August 13, 1761, and probated March 4, 1762. John Selby Martin, identified as a grandson of William Selby, was named as a beneficiary under the will.
A copy of the Will Book is available at the Maryland State Archives website:
http://guide.mdsa.net/series.cfm?action=viewSeries&ID=s538
[1760-1762 DD 1 Book 31, pp. 546-47 - msa_s538_000046.pdf, pp.546-547]