ClubSwim Solid Thin Strap One Piece Swimsuit in Black - it's a size 28 and is NWOT. I'm unsure as to the size conversion but it may be a Girls' 10/12 but can't be too sure.

The ClubSwim Solid Thin Strap One Piece Swimsuit is perfect for all kinds of training, competition, water aerobics and fitness swimming, this long-lasting one-piece suit offers the comfort and fit you've been looking for.

Round neckline
V back
Made without heavy-metal additives
Thin straps reduce tan lines
Large keyhole opening
Full chest coverage under arms
Fully lined front and back
Flat-locked seams
Seams lay flatter and closer to the body
Comfortable fit
Won't chafe against skin
Cut to sit low on hips for greater rear coverage
Long-lasting color retention
Fabric: 82% Polyester, 18% Spandex
Lining: 100%
Sun Protection: UPF 50+
Back Style: V back
Cup Support: No
Bottom Coverage: Moderate
Bust 11.5" flat
Total length 23.5"

#326