The difference between the 1901 (135x5) and the 1910 (135x9) needles is that the 135x9 needles has just a slightly shorter point by about 0.5 or 0.8 mm. Same other dimensions.

Sewing machine manufacturers use different needle style numbers for the same exact needles

*A needle system is a group of needles made to fit specific sewing machines. All needles in a system are of the same length and shank size, but are of various diameters. Lower size numbers are narrower or thinner needles used for lighter materials. Higher size numbers indicate a needle of thicker diameter, used for heavier materials. Match your needle system to your machine, then the size of the needle to the product you are sewing.
Specifications
# Singer Size 8 (Metric Size 60)
# Singer Size 9 (Metric Size 65)
# Singer Size 10 (Metric Size 70)
# Singer Size 11 (Metric Size 75)
# Singer Size 12 (Metric Size 80)
# Singer Size 14 (Metric Size 90)
# Singer Size 16 (Metric Size 100)
# Singer Size 18 (Metric Size 110)
# Singer Size 19 (Metric Size 120)
# Singer Size 20 (Metric Size 125)
# Singer Size 21 (Metric Size 130)
# Singer Size 22 (Metric Size 140)
# Singer Size 23 (Metric Size 160)
# Singer Size 24 (Metric Size 180)
# Singer Size 25 (Metric Size 200)