In the 1851 census he is in Glendower Street.

Also in 1841 census near Glendower St

There are/were a number of passages running off Monnow St that may have included a Grinder St - need to get an old map out just in case its marked :-)

http://ead.lib.uchicago.edu/rs3.php?eadid=ICU.SPCL.MANDEL&q=%22Samuel+King%22
Thomas Fletcher of Monmouth
Property on Bark Land and Grinder Street, Monmouth

<><><>
Glendower St is probably named after Owain Glyndŵr anglicised by William Shakespeare as Owen Glendower

http://www.historic-uk.com/HistoryUK/Wales-History/Glendower.htm

and the association with Henry V and Monmouth