As the title suggests, this is an Inokatsu M4A1 SOPMOD for sale. It is the Super version, meaning it has the Inokatsu M4 super bolt which has a removable weight, allowing for the user to have heavier recoil at the cost of higher gas expenditure or lower recoil using less gas, allowing for a full 50rd mag to be depleted on a single gas fill. Below I have copied and pasted the information direct from Redwolfairsoft, where the gun was purchased from for your convenience. I believe this is the correct listing as the name of the product and product code directly matches what is on the box (the invoice was misplaced). Click here to see the original listing on Redwolfairsoft.

The gun is in very good condition, with almost no external marks. There are very small scratches which are visible with close inspection, and a few markings from the movement of the bolt and insertion of mag into the gun. The gun is very, very solid, with no give at all around the whole body. The unique serial number for this gun is W233039.  

The gun will come with a single non-leaking Pro-win mag, which works perfectly given high quality BBs. The magazine will fail to feed if you try to use low quality ammunition, so make sure you use only the highest quality, 0.25-0.3g BBs. The buyer of this gun used it for filming purposes, so the gun was never used in a skirmish, never brought outside, and had minimal rounds put through. 

The gun was purchased some time in 2014, and the current listing price is over $1000, so I feel that £500 is a right steal for a gun of this calibre given you'd also have to import it from the US. They are no longer in production, so you're unlikely to find one sold new from a retailer, or one in as good nick as this second hand. I had the gun fully serviced by the reputable tech, Jon, at Wolf Armouries in London, who confirmed that the gun is in perfect working condition. 

Please don't hesitate to contact me if you are interested, or have any questions you'd like answered about the gun.

From the redwolfairsoft page:

With this version, INOKATSU has improved upon their previous 2011 version M4 GBB designs with an improved bolt for greater durability and more stable performance. The upper and lower receivers are also more refined with an improved fit and finish (tighter seam lines between the upper and lower receiver) thanks to a brand new mould. This is a direct improvement based on feedback and discussion between customers, Redwolf and Inokatsu. All improvement made are in an effort to maintain Inokatsu's premium dominance over other GBB rifles for realism, quality, performance and durability. 

The Inokatsu M4 blowback mechanism to designed to deliver an unparalleled gas blowback experience that generates good usable power, while providing customers with a choice of light or heavy recoil through the use of a removal weight in the bolt! 

The bodywork as usual is top notch, and with their 2014 version. Manufacturing process and coating of the receiver is the same as the real steel version (forged + CNC), making the Inokatsu M4 one of the most realistic looking airsoft guns available today. We showed the Inokatsu M4 to a group of Real Steel Shooters in the USA, and they could not see the difference between the Inokatsu M4 and a real M4! The color and finishing is spot on. Needless to say, each and every bit of its licensed body work had been expertly engraved into the body, the M4 SOPMOD of course has its own unique serial code. Realism even shows on the way the selector lever operates, such as not being able to flip the switch from semi / full auto to SAFE position unless the gun is cocked. This is the same as on the real M4. In the spirit of this being a training weapon, the barrel thread is the same thread and measurement as the REAL M4 for realism, but Inokatsu includes an adapter thread if you want to screw on your 14mm CCW (negative thread) silencer or other aftermarket airsoft flash hiders.

The 2014 Version of the INOKATSU M4A1 SOPMOD is built and assembled with exacting precision. The entire gun is solid with no wobbles or shakes making the Inokatsu M4A1 feeling just like the real steel. The Inokatsu M4A1 features a weight adjustable bolt carrier for full control of the shooting experience. At full bolt weight, the Inokatsu M4A1 recoil is similar to a real steel 9mm round with each trigger pull (kick is just as strong as previous version Inokatsu M4). On the semi-automatic firing mode, the INOKATSU M4 SOPMOD is able to completely fire off all 50rds from its magazine. But if you are a skirmisher and want a faster rate of fire rather than recoil, simply remove the weight from the bolt to easily finish 50 rounds in full auto. To maintain a perfect balance between projectile performance hard recoil, Inokatsu had to dial down the power from the originally planned 420fps to approximately 390fps. You may install a precision tight-bore barrel to increase FPS. 

If you leave the lead weight in the bolt and run it at full auto, you may get less than 50 rounds on one charge of gas but you'll enjoy a much stronger recoil for a more memorable shooting experience (rate of fire will be lower though). Regardless, each and every shot with the INOKATSU M4A1 SOPMOD would more than definitely put a smile on your face! The sound is loud and if you put your face against the gun for automatic fire, your ears will ring! If you intend to skirmish with the M4, then we recommend you remove the lead weight on the rear to lighten up the bolt for faster rate of fire and more efficient use of gas.

Includes:
4x RIS Rail Covers (Black)
1x 14mm CCW thread adapter
Total Length (Extended): 870mm 
Total Length (Retracted): 780mm