Thought I would post this in case anyone is interested
http://smith-wessonforum.com/smith-wesson-semi-auto-pistols/574751-numrich-1076-magazine-parts-1006-a.html
The spring butt plate catch from the 4013 4014 4053 4054 single stack series works fine in the 10mm magazines. Although it's shorter, it locks into the buttplate just fine.
I recently got some that were a bit too wide and had to shave a millimeter or two off each side. The length seems good: I wonder if the were intended for 4506, etc. Lord knows we have to be adaptable on this sort of thing.
Swede925