What's up with the LA Dart engine?

Today a buddy of mine told me about something called an "LA Dart" engine.. which was a lighter weight version of the 340 you could have ordered. According to him the numbers on the engine start with "A" in stead of some other letter and that's how you can tell one from a regular 340. Being a Dart guy, I was suprised I hadn't heard of any of this and I kinda doubt that's it's entirely true. Far as I know, there was only one 340 available, at least on my 70 Swinger.

Can anyone shed some light of this for me?

Corey