You may want to refer to the knitting instructions from 1918.
#8 Helmet Made in Two Parts
One hank of yarn (¼ pound); 1 pair Red Cross Needles No. 2.
The helmet is made in two parts, which afterward are sewed together.
Front of Helmet.—Cast on 48 stitches (11 inches), knit plain for 25 ribs (6 inches) and knit 2, purl 2 for 35 rows. On the next row the opening for the face is made as follows: Knit 2, purl 2, knit 2, purl 2, knit 2, knit and bind off loosely the next 28 stitches and purl 1, knit 2, purl 2, knit 2, purl 2. Run the stitches before the opening on a spare needle and on the stitches at the other side of opening knit 2, purl 2 for 12 rows. The last row will end at the opening, and at that point cast on 28 stitches to offset those bound off. Begin at the face opening of stitches on spare needle and knit 2, purl 2 for 12 rows. At the end of the 12th row continue all across to the end of other needle, when there should be 48 stitches on needle as at first. Knit 2, purl 2 for 24 rows.
Top of Helmet.—Knit 2, narrow (knitting 2 stitches together), knit 14, narrow, knit 14, narrow, knit 12. Purl the entire next row. On the 3d row knit 2, narrow, knit 13, narrow, knit 13, narrow, knit 11. Purl 4th row. On the 5th row knit 2, narrow, knit 12, narrow, knit 12, narrow, knit 10. Purl 6th row. Continue to narrow in the 3 places every plain knitted row with 1 stitch less between narrowings until 9 stitches are left.
Back of Helmet.—Work in same manner as for front but omit the face opening. Sew the stitches of upper edges together with joining-stitch. Sew up the side seams, leaving the plain knitting at shoulders open.