JBQS1933

I otograph of a groupresearching my family and I have come across a group photograph entitled ''SHELL CHECKING STAFF B SHIFT N.S.F.F. CHELWELL SEPT 1916''. My grandfather is in that photograph and I wonder if there is anyone who knows what a shell checkerf actually did. Perhaps there is someone else whose ancestor worked there during the WW1. JBQS1933