Many years ago when I purchased my 9mm Vollmer HK MP5, a #26 locking piece was included in the package price as an "extra". I seem to recall being told it would slow-down the rate of fire. I came across it last night in my parts bin and got to wondering about it. My reference materials tell me it was designed for .40cal so I'm not sure why the previous owner purchased it. I have not used it myself and it appears to be unused or slightly used. Several questions from the RKI on the Board: will it indeed slow down the rate of fire? Will using it damage or adversely effect the function in any way of my MP5? If I decide to sell it, what would be a fair selling price? TIA