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So, I was looking over my recent 3-digit Single Six acquisition to determine whether I should reverse the safety modification. Since the safety upgrade trigger is the wider version, it looks like the Ruger shop had to open up the slot by hand on the early frame so it would accommodate the bigger trigger.
This got me to thinking about the implication of that; does the change from large ALCOA G mark to a small one coincide with the introduction of the wide trigger? That would certainly explain the casting change but I have not seen it noted anywhere. Please pardon my ignorance if this topic has been beat to death elsewhere.
It also looks like someone tried to clean it with a harsh degreaser - strictly a no-no on the anodized finish.
Here's a pic:

That is a great observation.....you might be onto something....

RENE says small ALCOA G was introduced Oct 1957.
I did not find anything about a date going to the wide trigger.....

Thanks Rod.
The REF dies not mention a date for the change to a wider trigger, Jay, but in the next paragraph it mentions minor changes, which include "...the change to a thicker trigger by at least SN 97044" which was shipped in 1958.
I've wondered why Ruger would bother to re-do the mold for no apparent reason. The two frames look slightly different in profile with the top of the inside, below the backstrap, being slightly rounded on the later frame. But that is an insignificant change and doesn't fix any known issue. And the trigger slots are different in size.
If anyone has a gun shipped in 1957 with the earlier frame that would support the idea that the thick trigger 'triggered' (sorry about that) the need for a frame modification, which has a smaller ALCOA G cast into it. Just seems logical.
Owen

I'd thought this would have generated more comments. So, I changed the title to more accurately reflect the topic and I'm posting a photograph of the trigger slots on the two frame styles. The unfinished frame is the later, small ALCOA G, variation. Note that the bottom view shows that the later, thick trigger, opening is longer there, mainly due to a more severe "ramp" cast into the front of the slot.

Have I overlooked something? I hope to get educated if I'm off the mark.

Happy Independence Day, Ruger fans!

Can anyone with a mic provide slot measurements of single sixes and early flattops compared to later ones? I have a 29xx flattop grip frame I can mic when I get home and a 357 early gun also, but no more early single sixes. All mine are 1961+ in birth.

I dug through my loose grip frames and all the XR3 were the small G with the one exception already opened up to use a target trigger, so I didn't measure it. The three pictures below are XR3 small G, XR3-RED, and unfit Brass Frame.

Great info Troy, thanks for posting.
FT44

Yes, thanks, Troy.
My crude measurements with a caliper showed the width being very close to the same. The difference is in the length of the opening at the bottom of the frame, which is only about 2mm more in the later frame due to a steeper "ramp" cast into the front of the slot. Looks like they used a round file to remove material at the front when modifying an early-framed gun with the safety conversion.
Incidentally, both triggers are the same width at approximately .187 in.
O

It is curious why Ruger wouldn't use a conversion trigger that would fit all of them.
That trigger slot though is milled after the grip frame is cast, so perhaps they were slightly out of spec (narrower) on a few.
Chet15