This is a reverse stockinette stitch vest with the collar, front facings & armhole finishing worked in garter stitch.

This pattern is written with one set of numbers, the difference for the sizes is the gauge.

Gauge: for S/M 2 Sts & 3.25 rows = 1”; for L 1.75 Sts & 2.5 rows in Stockinette Stitch
S/M = 30-34” bust; L 35- 40” bust. Notes are included in the pattern for adjustments on larger sizes.

I used a #17 to obtain both gauges for the different sizes but a #15 or a #19 might be used too.

Techniques used: invisible (provisional) cast on, short row shaping and picking up stitches.

I would rate this for at least an advanced beginner. Good project to learn short rows since there are only a couple of turns.