Yes Claymore is right in that its the Marushin GBB shell ejector.
They are great fun but as they are getting on a bit now they can be quite problematic in terms of gas leaks/worn seals etc .
There are two gas ports ( one front and one back ) and one is for the Blowback and the other to propel the bbs so you can happily just use it as an ejecting model.
Nice and heavy for an airsoft gun If I remember correctly.
Shells are very hard to find and this model is quite harsh on them ...lots of snapped bits from the cartridge rims
on one of the auto sessions a round must hit something metalic as it sounds like a cash register making a sale
DOC