Mods:

Thumb gussset increase using M1R and M1L instead of KFB.

Thumb gusset instructions for 15 stitches is incorrect (only accounts for 13) so I added two more to the pattern on thumb gusset round 16 (where pattern says to place on stitch holder) and moved 15 stitches to stitch holder on following pattern round (round 5). Instructions say to M5 after slipping thumb gusset stitches when you only need to M3 to return to 20 stitches on the palm side.

On left mitt thumb gusset, I knit leaving two stitches on the left side of palm side to match right mitt gusset instructions. This means on round one of left mitt, I k17 before increasing stitches for thumb gusset.

Bind off in knit stitch.

For thumb, I picked up live stitches (15) plus 5 (20 total): one stitch from each ditch and the right leg from each of my M3 backward loop stitches. On first round of thumb, I knit the ditch stitches together with its respective neighboring stitch from the picked up stitches above the thumb hole to help close the gap (leaving 18 stitches total), per the instructions found here. K7 rounds total, bind off in knit stitch.