Twist vs. Plain Twist

[Jim Kremmel]

I’ve spent the evening here looking for examples or threads discussing Twist vs Plain Twist barrels. The info under the technical info tab indicates that there were many more plain twist barrels fitted than twist barrels. Here are some pictures of my PH sn 144,983. I see a T stamped in the barrel near the weight, but no T with crescents. Which type do I have, and what is the difference/significance between the two types? My apologies if this has been discussed previously, I don’t yet have any of the Parker reference books.