Low to mid carbon steel would be ideal, and to my way of thinking the introduction of vulcan steel seems to be around the time you stop seeing the striations, which is why I was guessing 1900.
For the record, cast iron has a high carbon content while wrought iron has a relatively low almost nominal carbon content. Since iron is not completely homogenous and varied in composition from batch to batch, some iron had more carbon. For that matter, some iron would likely show striations and some might not, depending on how fine the quality was. Additionally, two billets from the same iron source (or steel for that matter) might show differently depending on say the forging temp so im with you on process innovation.
It is also possible that those receivers didnt show lines as clearly when they left production but as the case wore off some metal eroded and the lines showed. wire wheeling or naval jelly or similar could also do it.

The graining lines in the frame are very common or typical. It is usually more visible in older guns and ones with a lot of surface wear and light corrosion.