OP, first things first. Are you certain your 64-5 is DAO? Many non-factory nubbed or bobbed hammers actually do haver their single action notch intact.

Factory bobbed hammers do not have the single action notch.

Which do you have?

Assuming your 64 is truly DAO, the easiest way to restore it to single action capability is to get a correct-era K-frame frame hammer with the single action notch, swap the double-action sear from your current hammer into it and install the "new" hammer into your 64 and hope it's a drop-in fit. They aren't always.

Alternately, yes, the hammer and trigger combo from a like-era Model 10 (or most centerfire K-frames) could be used. Again, they may drop in, or they may require fitting. And even if they do drop-in and the gun does fire, that doesn't always mean the lockwork is functioning optimally; back in the day these guns were hand fit at the factory to spec, and that takes inspection (and if necessary, correction) by someone with the right know-how and tools.

Lotta folk do just swap parts and if everything seems to work they call it good. Your mileage may vary.

The want-ads on this forum, eBay, and Jack First and Numrich are still your best bets for older S&W parts.

To repeat: make sure you're getting parts correct to your era S&W.

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Pickpocket