haha, two clicks later i've found this one - it looks perfect http://workholdingcomponents.mortonmachine.com/viewitems/thumb-screws-metric-thumb-screws/metric-precision-stainless-steel-thumb-screws?
These people may have what you want
I purchased several of this item when upgrading my first rail kit.
I am not saying that is the screw you need, you'll need to determine the thread etc
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