A contact piece for a 93 is going to have a slightly different angle compared to one for a 94. Either one will work, but because of the angle difference working against the proper paddle will cause problems. For the longest time, I had a 93 contact piece in my 94 and although the paddle worked, it always had a rough bind through the operation. Once I realized there was a difference, I got the contact piece swapped out and the release now worked like butter and had a positive snap back to it.
Here is a photo someone posted on the site showing the differences. You can modify a 93 to work but, you'll have to shorten the shank of the contact piece as the MP5/HK94 is a little shorter. You'd also have to rework the angle to a better degree.
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