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. The 1910 & the 1901 are not the same although they are fundamentally the same.The only difference is the length of the point & of the shank There used to be more of a difference as the needles were changed from what they originally were.The 1901 used to be the same as 135x7(DPX7). This was when 135x5(DPx5) was slightly different than the 135x7(DPx7) When the 135x5 & 135x7 were made interchangeable with each other the 1901 became interchangeable with the 135x5 & 135x7 too The 1910 are the same as 135x9, 134K & PF797 Singer got away from using the 135x9 on the 20U and started using the more common 135x5/135x7. Diamond Organ Needle Co.

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.
# 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)