Shetland 2 ply question

I’ve got a ‘Knitting in Vogue n2’ book printed in 1984 and I was thinking of doing the cardigan part of a twin set.
It’s a 1958 pattern and calls for the following:
ANI Shetland 2 ply 19x25g
1 pair 3mm needles
1 pair 3.75 needles
The given tension for 10cm is 26 st and 34 rows.

My question, as a beginner, is this: I always thought 2 ply was very fine, good for baby clothes etc. The photo of this cardigan in the book seems to show a more robust wool. Is that because it’s Shetland? Or maybe those ‘ply’ terms have changed over time?

Ply isn’t really a very reliable yarn descrption since the size of the ply can vary. The gauge or tension gives you a better idea of yarn weight and yours is in the sport/baby yarn category.
http://www.craftyarncouncil.com/weight.html
https://www.lionbrand.com/cgi-bin/yarnInfo.cgi?yarnByClass=1

Check also in the pattern description that the yarn isn’t held double.

Had to uhh we the way back machine, but here’s why ply isn’t helpful.
http://web.archive.org/web/20061222113331/http://www.purlbee.com/2-ply-4-ply-why-ply/

Thanks for the info. You can see how long I’ve been away from knitting. There was a time ply meant something!

I am also about to knit a 1950s Vogue pattern but this one called for “3 ply” and gave the tension in terms of stitches and rows per inch. I bought a number of yarn samples and made some trial swatches. In the end I chose a 4 ply Alpaca, which seems to as thin as the originally specified 3 ply.