I am wondering what this notch was used for on this crossarm. Also wondering if this is part of a crossarm or a compete small crossarm.

ANSWERS: I received many different answers on what this "notch" was used for. Listed below is a summary of the answers I received.

Brent Burger called the notch a "gain" and said it was there to hold the crossarm perpendicular to the pole when the bolt is tightened through the two.

William Shelton said that a metal washer and lag bolt would be used to hold the piece on the pole or what ever it was attached to.

Jim Harlow said that the notch butted up to the pole. Most likely a metal pole or brace and not the standard wood pole.

Donald Woods said that he has never seen a crossarm with a gain in it. He suspects that my crossarm was originally a 10 pin crossarm and was mounted onto a metal pole or bracket. The crossarm was maybe used on a RR bridge.

Thanks to everyone who answered my question and gave me information about the crossarm.