  The fourth Fermi Large Area Telescope Source Catalog (4FGL) lists over 5000
gamma-ray sources with statistical significance above 4$\sigma$. About 23% of
the sources listed in this catalog are unidentified/unassociated gamma-ray
sources while ~26% of the sources are classified as blazar candidates of
uncertain type (BCUs), lacking optical spectroscopic information. To probe the
blazar nature of candidate counterparts of UGSs and BCUs, we started our
optical spectroscopic follow up campaign in 2012, which up to date account for
more than 350 observed sources. In this paper, the tenth of our campaign, we
report on the spectroscopic observations of 37 sources, mostly BCUs, whose
observations were carried out predominantly at the Observatorio Astron\'omico
Nacional San Pedro M\'artir and the Southern Astrophysical Research Observatory
between August 2018 and September 2019. We confirm the BL Lac nature of 27
sources and the flat spectrum radio quasar nature of three sources. The
remaining ones are classified as six BL Lacs galaxy-dominated and one normal
galaxy. We were also able to measure the redshifts for 20 sources, including 10
BL Lacs. As in previous analyses, the largest fraction of BCUs revealed to be
BL Lac objects.
