Double check if it is not a #28 (suppressed MP5K-PDW and/or with 147gr ammo), which I have used to slow down a full size MP5 (have less energy reserve for when dirty or weak ammo, so don't use for anything critical). The cost of an HK #28 is fairly high now days, over a $100 new. #28 Locking Piece for MP5K - HKPARTS.NET
The #26 appears to have been the 40 Cal locking piece before / instead of the current listed (HKPARTs) Hi and Low impulse locking pieces.
HK Locking Piece Table What is interesting is both the #26 and #28 locking piece are 80 degrees.