Howdy
There has been no half cock notch for loading on the hammer of a Ruger for over 40 years. When I bought my Blackhawk back in 1975, I did not even know a half cock notch existed on Colts because I had never handled a Colt at that point.
I was fat dumb and happy, using my Blackhawk for over 30 years. Yes, it can be a bit annoying when the cylinder clicks past the loading gate and you have to go around again to load that chamber, but it certainly is not the end of the world.
What happens is, if you carefully rotate the cylinder to line up a chamber with the loading gate, everything will be fine. But if you over rotate just a teeny amount, the pawl (hand) pops into the next ratchet tooth and there is no way to back up the cylinder to load or unload that chamber without going around again. I did it for over thirty years without pulling out too much hair.
There is no half cock notch on a New Vaquero hammer either, and I'll bet you a donut there is not one on that Flat Top you are interested in either.
Here is a photo of a New Vaquero hammer. Notice there is no 'safety cock' notch and no half cock notch. That small cutout near the hole for the pawl is the full cock notch. That's all you get.
What Ruger did to remedy this situation with the New Vaquero was they installed a spring plunger in the frame. The spring plunger is the round button looking thing near the cylinder pin hole. The plunger is positioned so that as you rotate the cylinder, the plunger will engage the ratchet teeth, keeping a chamber lined up with the loading gate. Got one in hand right now verifying this. With the loading gate open, the hand is completely withdrawn into the frame. What you hear clicking as you rotate the cylinder with the gate open is the spring plunger popping up between ratchet teeth. Nothing at all to do with the hand or any notches on the hammer.
I will bet you another donut that Ruger did the exact same thing with that Flat Top, since the frame is basically the same as a New Vaquero, with the addition of adjustable sights. No half cock hammer, just the spring plunger popping over the ratchet teeth as you rotate the cylinder. It does work very well, the plunger aligns each chamber perfectly with the loading gate.
Here's another rub. If you want to install a Free Spin Pawl, you have to remove that spring plunger from the frame, or it will prevent backing up the cylinder.
Why didn't Ruger update the frame of the full sized Blackhawk to include the spring plunger? Probably because they have sold bazillions of them, and it really isn't a big deal to rotate the cylinder carefully so you don't over rotate a little bit. Go around a few times, and you will learn to do that.
----------------------------------------
Here is another option you may or may not like, because it is expensive.
You can buy an after market hammer for your Blackhawk that has a half cock notch on it. I have installed one of these half cock hammers in three of my 'original model' Vaqueros, that had the exact same annoying habit of over rotating because their frames were the exact same as the Blackhawk. Except for the adjustable sights.
Here is a photo of one of my 'original model' Vaqueros with the its hammer at the half cock position. Notice the chamber is perfectly lined up with the loading gate. With the hammer in this position, the cylinder no longer over rotates that teeny amount, you simply put the hammer at half cock and load exactly like a Colt. No, there is no 'safety cock' notch, just the half cock and full cock notches. So you don't get four clicks like a Colt, only three. (The middle click is the bolt popping up.)
Here is a link to the Power Custom half cock hammers for Rugers. Click on the photo to expand it and you can see the half cock notch up near the spring plunger of the hammer.
http://powercustom.com/store/index.php?main_page=product_info&cPath=12&products_id=136
Note: the machining of these hammers is actually better than the original stock hammers. I can testify to that. The originals are castings. The Power Custom hammers are cut by Wire EDM, and are more precisely made than the OEM hammers. Also, if you buy the kit, and install one of the lower strength hammer springs, it will indeed reduce your trigger pull, because the heavier the hammer spring, the more friction the sear has to overcome sliding out of the full cock notch. I recommend the mid strength spring (18 pounds). That is what I put in all my Vaqueros. Also, don't cheap out and cut up your trigger like the instructions suggest. Spend a few more bucks and buy the Power Custom trigger that is already shaped properly for the half cock hammer.
This solution really does work, and you will get the half cock hammer that I'll bet you a donut you will not get with that Flat Top Ruger.
Or, you could try to find an original Three Screw Ruger like this Three Screw 44 Mag Flat Top that really does have a half cock hammer.
Happy Hunting.