On one site, he is recorded as Reginald John Isaacs.
The householder is Sydney Arthur Isaacs so that seems to be where the error in transcription has occurred.
There are some trees which show Harriet as born 1880 in Norfolk. This seems to be wrong.
There is a Harriet Dewing 1895 in Devonport, 1901, with parents Charles and Harriet.
In the 1921 entry found by heywood there is also a visitor in the household aged 11 with a surname very similar to Harriet Pope's maiden name.
I think that person was Harriet’s sister.
Possible marriage of Harriet's sister (not given full names as have not established deaths)
Miss
D M DewingMarriage 27 Jun 1931 Plymouth
Parents
Charles James Dewing; Dewing
Spouse C D Barrell
Unfortunately father's not named on this entry
Name
R.J. PopeMarriage 27 Aug 1916 St Thomas Coventry
Spouse
Harriett DewingUK, British Army and Navy Birth, Marriage and Death Records, 1730-1960edit to add
Harriet Dewing Birth 15 Jan 1895
Baptism 15 Mar 1895 Devonport, St Mary, Devon, England
Religion Anglican
Father
Charles James Dewing Mother
HarrietNo image transcription only on Anc*y