Parker Research Letters & available serial numbers

[MrBojangles]

Is there any rhyme or reason as to what serial numbers are available for research and can be made available in a letter and those that are not?

Are certain blocks of numbers available? Were some lost or destroyed?

Much to my amazement, a Parker I swear I had typed into available serial numbers said the number wasn’t available awhile ago and today it is. And so I shall order a letter.

Which leads to the question have more records been located over time and this is why my Parker was not available for research previously? If not I probably typed the number in wrong previously in true dyslexic fashion.