These screws are designed by Magnum Airpower LLC and custom manufactured. 

 This custom-made Torx-drive stainless steel screw is an exact-fit replacement for the original inner breech screw on all 1377, 1322, 2289, 2240, 2250, 2260, 2300, 2400KT, 1300KT, Discovery, Maximus and similar models. The Torx socket fits a T8 driver which is not included.

 These are 410 stainless steel heat treated to meet metric class 12.9 with a specified hardness of HRc 39-43. The approximate tensile strength is in the 177,000 - 196,000 PSI range.

 These screws require a T8 screwdriver. Do not over-tighten the screw, snug it down only until the breech is firmly against the tube with no wobble. If you try to tighten the screw until it stops turning it will strip out the threads in the tube.

 INNER BREECH SCREW LOCATION: For Discovery, Maximus, 2300 and 2400 models it's underneath the bolt, if you pull the bolt all the way back you can barely see it under there. You have to remove the rear breech screw and rear trigger frame screw to take off the rear tube plug. Then remove the bolt handle. Now you can push the bolt back far enough to expose the inner breech screw. I suggest putting a very light coat of medium strength threadlocker on the bolt handle when you put it back on. They tend to come loose otherwise, you only need to put it on finger tight, don't use a wrench, easy to damage the threads. The standard Crosman Steel Breech Kit, 13XX and many other models have the screw in the position shown in the photo.