Emma, it isn't a question of oral tradition taking over.

Websites have a horrendous habit of regurgitating the mistakes of other websites.

In this case, the phrase is "SHUTTERING JAMBS" and the jambs are the reinforcing members, usually of folded sheet metal on large pours as in a dam, which are bolted round the sheets of sheathing plywood to prevent the shutters from buckling outwards under the pressure of the concrete while it is being poured and vibrated.

End of discussion.