Gauge is slightly off but measurements seem big so doing size 6/9m nonetheless which should come out slightly smaller and hence just the right size.

Pattern adapted for top-down, raglan lines to align with armholes. Trying jogless stripes.

Plato held double.

Buckingham on 3.5mm, Plato on 4mm except for ribbing on 3.5mm. Bo on 5mm.

Gauge
Buckingham 3.5mm 27 * 34 // 25.5 * 34
Plato 3.5mm 28 * 38 // 27.5 * 36 & 4.0mm 26 * 34 // 25.5 * 35
vs pattern 24 * 32

Knit Notes
Crochet co 84sts on 3.5mm.
Garter edge throughout.
2x2 ribbing for 7r, setup [k2, p2]*, k2, p1, k1.
Next row (rs) start st on 4mm and inc ev 6/5sts.
Place markers at same time: (1+13)-22-26-22-(13+1).
Raglan inc ev other row 14x (1-27-50-54-50-27-1).
On next rs row set aside sleeve sts, co 4sts (pick up 2 add sts in holes on next round).
54r for body.
10r 2x2 ribbing, see above.
Bo loosely in patt on 5mm.

Striping sequence body:
#1 7r ribbing + 8r
#2 14r
#3 8r raglan + 6r straight
#4 14r
#5 14r
#6 14r
#7 4r straight + 10r ribbing + bo

Sleeves: 50sts + pick up 8sts.
Dec ev 3/4r 11x.

Striping sequence sleeves:
#3 8r raglan before sleeve separation + + 2r (pickup + 1) + dec + 2 + dec
#4 3 + d + 2 + d 3 + d + 2 + d
#5 3 + d + 2 + d 3 + d + 2 + d
#6 3 + d + 10
#7 1r of st st + 7r of 2x2 ribbing + bo

Button bands: 74sts (11 per purple, 10 per green) 2x2 ribbing, 7r + bo.
7 buttonholes, one per stripe.

Final measurements: length 28.5cm, width 24cm, length from underarm 15cm.
28.5gr of Buckingham, 42.7gr of Plato.