Howdy
Funny you should ask. I happened to have my old 45 Colt/45 ACP Blackhawk that I bought brand-spanky new in 1975 completely torn down for photos for another thread.
This old revolver is completely stock, I have not replaced any parts over the years.
I never noticed before, but you are correct, the transfer bar is thinner than on more recent models.
Here is a photo of the Transfer Bar. The arrow is pointing to the surface that gets struck by the hammer.
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Here is a different view. Again, the arrow is pointing to the surface that gets struck by the hammer. The lighting in this photo reflected the wear area on the Transfer Bar where it has been struck by the hammer over the years.
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Still another view. Notice how the sides of the upper section are parallel.
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Here is a photo of the transfer bar with the hammer at full cock. Sorry there is a little bit of grunge in the photo, and the rear sight has some wear on it because I have had this revolver for 45 years.
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This is the transfer bar on an 'original model' Vaquero. Sorry I cannot tell you exactly when this one was made because I have two of them. One shipped in 1993, the other one shipped in 2000. I don't know which one this is.
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This is the transfer bar in a New Vaquero. I bought it new in 2006. Notice there is a thicker section up top where the hammer strikes it.
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In this side view you can see how the transfer bar in the New Vaquero is thicker at the top. These photos should give you some idea of the difference in thickness of older and newer transfer bars. Am I going to take them apart again and measure? Absolutely no way. If you have ever taken one of these apart you know what a pain it is to get them back together again.
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Does it matter? I have had my Blackhawk for 45 years. I have no idea how many rounds have been through it in all that time, Probably not bazillions because it sat unshot for about 20 years. But probably a couple of thousand. Knock on wood.
Just so you know, transfer bars still break in Rugers. Not often, but guys in CAS who shoot thousands of rounds every year sometimes get broken transfer bars in their New Vaqueros. That is with the latest design of transfer bar with the thicker section at the top. Some guys even go to the extreme of removing the transfer bar and welding up the hammer face so it will strike the firing pin without a transfer bar, just so they won't experince a broken transfer bar disabling a revolver during a match.
Even with the newest transfer bar design, the culprit behind broken transfer bars seems to be 'transfer bar pinch' which has nothing to do with how thick the transfer bar is.