The KAC vert grip and some of the knock-offs attach with a post that screws upwards.  They also have witness holes.  When putting them on a FIRSH or M44, they don't hold B/C the rails are relieved down the center.  

I need a bolt that goes through the witness holes to secure the grip (actually, two grips) Ideally it would have a stop at one end, a squared body, and threading at the end to accept a thumb screw.  I'm thinking that maybe a scope base manufacturer has just that item, but I'm having trouble finding just the right part.  
Suggestions?
Sources?

Well you could get a cheap set of Weaver rings and rob them of their cross-bolts...I looked in my Brownell's catalog and I didn't see any of them sold as replacement parts....

Now there is one way to get them....call Weaver and tell them that your cross-bolts on your, say, Weaver 4X4 Split Rings got stripped and see if they will send out new ones....hey that worked with me on my Sig P225 when I wanted a new sent of grips...told Sig that they had cracked and they sent out a new set.....

Then you can see if that would work...I checked my spare parts bin and I don't have anything like that or would give you one...if I come across one I'll let you know....

if you put a bolt thru the witness hole, what happens to the small post in the middle of the locking screw of the grip?  are you going to cut it off, not tighten the locking screw all the way (it'll be too loose) or just leave it off the grip?
i encountered the same thing when putting a KAC grip on my SIR - it has a center channel too, so the post has nothing to engage.  i cut a couple of short (about 1") pieces of plastic tie-straps which just happened to be the right size to lay in between the cross rails.  knowing where i wanted the grip to be centered, i put the two pieces of plastic on the crossrails, then slid the grip over them, tightening the locking screw so that the post was now between them.  the tie straps on either side of the post prevent it from sliding forward and aft.
that's my fix until the tangodown vertical grip comes out in about a month to replace the KAC ones that i have.  
cheers,
MM

I'll probably cut the post off.