This first picture is shown just to give an idea where the rear of the frame is in relation to the tumbler mortise.  The left side of the ruler is the rear of the frame line.

In this last picture the mortise is complete.  The tool path I cut in error is shown but, now at the front of it is a round pocket.  The tool path and this pocket are now the same depth at .040.  The round pocket is where the trigger pivot screw will be.  The trigger was to have a .010 boss on each side, but now the side where I messed up will have one that will be .050 to make up for the error.
Bob

Once the pin hole and back slot were cut I then moved to the side slot.  I have never actually seen a transfer bar style firing pin apart, so this is my version of what I have seen in pictures.  Since the bar needs to move in and out with the firing pin, but needs to be restrained in the breech block, the side slot will have a key machined on the top and bottom.  This will allow it to slide in and out but keep it in place in the breech block.  A small screw from the back side will be adjustable to stop how far it can back out.  And the firing pin will be a separate part form the transfer bar.

This first picture is the setup used to machine the slot and keys.  I ran the indicator in the back slot to get it exactly vertical in the mill and then double checked it with a pin placed in the firing pin hole.  I used also used the slot and pin to setup the angle matching he pin angle, I simply angled the vice to get the correct angle. 

I ground a carbide 3/16 end mill to the width I wanted for the slot under the keys, .080.   Photo 2  The slot between the keys will be  .112 so the shank on this ground tool is .105.  The cutter is 3/16 in diameter.  

Phot 3.  Might be hard to see but the first step and inside width are  now complete to depth in the breech block, less a couple thousands that will be finished in the final step in machining the slot under the keys use the tool I made.

Photo 4.  The transfer bar machining is complete, except for a slot for the riser off the transfer bar that the hammer will actually strike.  The keys and under cut can be seen.  The sharp inside corner will be radiused, probably with a file.  The transfer bar will be radiused also, to match and add a small amount of strength to this area.

Bob

Continuing with the breech block, firing pin and transfer bar this first picture is showing the new transfer bar still attached to the supporting steel but fitted to the breech block.

The second photo is the bar cut free.  I actually ended up making a second transfer bar because the first one was just slightly looser than what I wanted.  At this point the screw coming in from the rear that limits the travel is made and installed for testing it and eventually for assisting in the trimming of excess metal from the transfer bar so that it fits the breech block.

In this third photo the excess metal is gone and the breech block is installed.  I removed the excess transfer bar metal in the mill.  I locked the bar in its furthest rear position with the screw and started picking away at it with an end mill, which got me within .010 of the final size.  I finished it with files. The bar was a little sticky in the breech block so I lapped it in with 600 grit lapping compound.  

I have not made the firing pin at this point, I want to ream the pin hole a couple thousands bigger and am waiting for the reamer.  My intent is to have a spring under the front of the firing pin that pushes it and the transfer bar back.  The spring I have is slightly tight in the hole so I need that reamer.

The last two pictures are the finished breech block.  The retaining screw can be seen.  By adjusting this I can limit the rear travel of the transfer bar without interfering with the forward travel when the hammer hits it.  The the hammer lug will be cut down after the hammer is fully installed so they can be properly mated.

Thanks!!   
I have to admit, this just about had me throwing the whole project in the scrap bucket when I discovered the first transfer bar was loose and had to make it again.
Bob

On to the hammer.

Rather than starting with steel, knowing there would be considerable filing required, I decided the first one would be from plastic.

This first picture is a poorly drawn first version on the plastic.  It morphed into a little different shape as I progressed, and the steel version will be changed even more so.

The last four pictures are the hammer shaping and fitting to the frame and breech block.  I think I will be able to lower the top of the hammer after I get the frame radius around the the breech block lowered, making it a bit lower profile hammer.

Bob,
Not for a moment did I believe you would toss this project  it's one of those now that you are committed to.   
I didn't pay any attention to the internals of the original I had in my hands, I'll look next time... the '74 has a small protrusion on the front of the transfer bar that fits a recess in the mortice when the block is at battery.  When the breech block begins to move the transfer bar pushes the hammer back off the firing pin.  Just something else to look at and ponder    You may have to make another transfer bar 
(I wouldn't )

Greg

Needed to get at the hammer but, before that, I needed the cartridge tray machined in the frame.

So, this is just a series of pictures showing the progress.  That's a 25-20ss in the tray.  Also showing is the machined cut-out for clearance of the hammer nose.
Bob