The m50 is made by origin jumpworks, but not in the 300 series. The 300 series racer is the 350R. In-game lore says people were disappointed when origin (known for fast ships) released the 350R as a "racing" ships that was really a faster luxury ship. (The 300 chassis is really pretty large for a racer) In response origin put out the much smaller, much lighter, M50. It's 11m long and weighs 12k kilos while the 350R is 24m and weighs in at 20k kilos.
Yeah, I'm still a bit disappointed in my 350r, but I didn't buy it to be a racer. Instead, it's going to be used as an information runner and high-speed interception for my outlaw.
That said, I'll be grabbing an M50 in the PU as soon as I can to have a dedicated racer.
Oh, and to anyone disappointed with the Mustang Delta, just keep using it. It is very unique as a fighter in that it doesn't do any one thing exceptionally well, but between the rocket pods and its low signature, I've been having great fun with it. Playing chicken with Hornets, launching a rocket volley, then peeling off at the last second has worked very well for me. The other thing is its agility. For a dedicated fighting vessel, its size and maneuverability are great. Getting up alongside Hornets and Avengers then dropping behind, popping rockets, and breaking off is fun, too. Those rockets pack one hell of a wallop.
EDIT:
I guess I can play with the Delta, since non of the others are not as good or par with my omega in every other aspect.
If you go into it not expecting it to handle like anything you've used and can get used to the dumb-fire rockets, you'll love it. So far, it's my favorite ship in AC. It really feels like a breath of fresh air being that it doesn't feel like any other currently usable ship. It's also fun to see guys in Hornets and Avengers in chat getting upset that they get downed by a Mustang.