That is a very unusual mount but clearly very effective since he discovered 10 comets from 1873 through to 1912:
19P/Borrelly, C/1873 Q1 (Borrelly), C/1874 O1 (Borrelly), C/1874 X1 (Borrelly), C/1877 C1 (Borrelly), C/1889 X1 (Borrelly), C/1900 O1 (Borrelly-Brooks), C/1903 M1 (Borrelly), C/1909 L1 (Borrelly-Daniel), C/1912 V1 (Borrelly)
I assume that they were all with this instrument but I haven’t checked.