Well, a Model 400 it is then. No Greener crossbolt, its a screw grip for sure. A vintage gun site in the UK agrees that 300's
were all made with the Greener cross bolt.

Even though it's chambered for 2 3/4 inch, 1 1/4 oz loads I suspect a prudent owner should probably stick to low pressure shotshells like RST's 2 1/2 inch, 1 oz loads. I have no experience with their 2 3/4 inch shells.
Tom