A very nice collection! While not necessarily "a huge gaping hole" I think a 4" or 6" .357 mag revolver would make a nice addition to your collection. Since you don't have a Ruger in the bunch, a GP 100 would be my choice. I don't have very deep pockets and appreciate the value Ruger offers. A 686 S&W might go well with the 67 though. As already mentioned, a friend has the S&W model 17 Masterpiece and it is the sweetest shooting revolver I have ever handled. It is worth a look.
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"If you let "need" be a requirement and Government be the arbiter of that "need", then Liberty is as dead as King Tut." Jimbob86