Hi there,
Welcome to this fun, nutty, crazy hobby we call insulator collecting.
I'll take a crack at answering your question. When I started collecting insulators more than 30 years ago, I too was fascinated by the variety of numbers around the <D> on the Dominion-42s. 20 plus years later, once I got more seriously into collecting and got my first price guide, I finally learned what those numbers are.
The numbers and their positions around the diamond represent a mold number and/or a date code for when the insulator was made. No documentation has ever been found from early Dominion production to explain exactly what the code was or how it worked. Other manufacturers used similar (but not the same) codes with numbers (and sometimes dots) to indicate the same thing on their products.
Some detail-oriented collectors may try and collect all possible mold numbers of a given insulator style, but generally, the different mold/date codes have no bearing on the value or desirability of a particular insulator.
What does usually matter is style, main embossing, colour and condition.
Hope this helps!