When I was shopping for my lefty I just called Beretta customer service and asked for a list of calibers. The product numbers for Lefty in Light Stainless start with "JRTB4##" where right-hand T3 Light Stainless are "JRTB3##".
Here's a list of offerings For T3 Light Stainless Left-Hand, followed by the product number if known, when I last checked about 8 months ago:
30-06sprg JRTB420
.270wsm JRTB440
.300wsm JRTB441
.300win mag JRTB431
22-250
.308
7mm rem mag
.243win
You'll get the most accurate info by just calling them.