Caterpillar twenty serial numbers

Hi I’m looking for a breakdown of the caterpillar twenty serial numbers and date of manufacture if anyone has a good one, possible with a breakdown of when the PL models started to mix in with L models
I attached a picture of the operators manual showing some things exclusive to L-1 to 340-350 that would make me wonder if this is the time that The first pl was introduced and possibly an upgrade to L351 

thanks
Paden

Available from ACMOC serial number referance book 1925---1960

PDS, where are you located? I have a parts 20 that I will sell cheap. Missing carb and mag and a few other parts.. It is in the L600 range. Grant.

BC Canada 

if you’d like text or phone  250 264 7545 

thanks 

 

chapter 12 website has a listing:
https://chriscomachinery.com/caterpillar-machinery/#caterpillar-model-twenty