Here is the response I received a couple months ago from Imtra when I asked a similar question:
Good morning Ken.
Thank you for your inquiry.
A 133 stamping on the gypsy denotes that the gypsy will handle 5/16” HT (high tensile) chain.
The stamping for 3/8” HT chain is a 112 stamping.
The gypsy part number is our MPH1200112BC. List price is $675.00. Your discounted net is $506.25. This gypsy is in stock.
I have attached a credit card form if you would like me to get one going your way.
Enjoy your day and let me know if you need anything additional.