Well, I found a steel tool at a rummage sale, and the ends look eerily similar to a park brake tool that is no longer available.
The BT3 brake bending tool is for the older aluminum brake arms. They need to have a slight toe-in on the front edge of the pad to brake properly. Being aluminum the arms are easily twisted out of whack by any number of things. While many mechanics use an adjustable wrench to re-bend the brake arms Park came up with a nicely made wrench to do the job. Since it's been discontinued, I've tried to find one reasonable and haven't been able to.
Enter my piece of steel. 11 1/4" long and 3/8" thick. The two end openings are 9mm and a tad over 5.5mm. Asked on the bike forum if anyone with the BT3 tool if they could measure the opening. A member responded and said he measured 5.6mm. So I'm in business.
I think the chunk of steel was worth $1. Can't figure out who made it or what's its OEM purpose was. Though I suspect it had an auto brake function. Say a disc brake rotor straigtener?