Printing lines themselves are fairly new, with general use dating from maybe the late 80s. Paperbacks might not have started using them regularly until the 90s. I wouldn’t expect to see them on a 1961 book, even in paperback, unless the reprinting was extremely recent.
It’s not true that they were common in the lead type era. Almost all the older books I know of spelled out the words 6th printing or something similar. A few publishers used a letter line instead of a number line, though, in hardbacks.
Pocket Books was unique, to my knowledge, in providing the entire printing history for some of their books. For a popular mystery, that might use up an entire page. Take Ellery Queen’s Double, Double, because it’s the first one my hand fell on.
Little, Brown edition published June, 1950
1st printing … April, 1950
Dollar Mystery Guild edition published October, 1950
1st printing … October, 1950
2nd printing … June, 1951
3rd printing … July, 1951
Condensation published in Toronto Star Weekly in 1951
Pocket Book edition published July, 1952
1st printing … May, 1952
The Pocket Book printings could go into the dozens.
Those were absolutely invaluable for bibliographic purposes. They also showed how formal publication dates and printing dates varied, which give an understanding of how points could arise. Unfortunately, they stopped doing them in the early 50s.
Some publishers did do a complete history of month and year of printings for hardbacks as well. There are thousands of publishers, and each had a house style.