Most products in the industrial sector have straight forward naming conventions that can be easily understood by looking at the specification sheets of the related components. Be it max torque, length, outside diameter... there is usually some rhyme or reason to the sizes of a given product.
The secret lies in history. At one point in time, remember Lovejoy first launched the jaw coupling in 1927, the number signified the max bore of the jaw coupling. A L035 had a max bore of 0.35 inches, and a L150 had a max bore of 1.5 inches. Over time, given technical advances of steel and rubber technology (Lovejoy's elastomeric spiders were original not rubber but layered leather), the max bore of these coupling hubs increase while the components themselves maintained the same external dimensions.
Given the product dimensions remained the same (other than they could now be bored out larger), Lovejoy opted to maintain the legacy naming structure... and not rename it every time technology allowed the performance specs of the product to be increased.
Today a L035 has a max bore of 0.375 inches and the L150 has a max bore of 1.875 inches.