I live in SoMd myself--North Beach--and am a former carpenter. Spanning 7 feet of fireplace doesn't sound like too much of a problem. Unless of...
from your pics: Your curve is made up of 9 pieces with the plywood's center line splitting middle piece. Try using 8 curved pieces instead, with a...
Remove a curved piece and rotate/spin the new section as needed.
I'm thinking it's a holddown for somekind of conduit. Or a cable clamp maybe for a bridge.
On wooden boats, a small piece of wood was carefully inset/inlaid to repair a spot of rot. Doing this instead of replacing the entire board...