Judging by pictures of new sintered bronze plates I saw in the wests catalogue online, I'd say it's sintered bronze but it is now the same colour as a zinc anode. I don't remember the details of the shape other than that it was about 1/4 to 3/8" thick by about 6" long and maybe 2" wide with two holes for the bolts.
Got it cleaned up and sure 'nuff, this is what it is. Re-installed it yesterday. Now I gotta get that engine fired up today. The bottom gets painted this weekend and hopefully will be in salt water on Monday.