I worked in an historic building whose alleyside wall fell into extreme disrepair. We had an architect create a set of plans to exactly duplicate its look including using the original bricks. SHPO said "N." We had to replace the historic wall with a concrete block wall that was the ugliest thing you could imagine so that the public wouldn't be confused.
We got SHPO thought, a large window above our front door, on the streetside had a rotten sill. We pointed out to SHPOO that if the sill failed it could kill someone. SHPO said that we could replace it as long as the mullion pattern matched exactly its historical form. We were able to produce historical photographic records showing FIVE different mullion patterns in the "historical" period SHPO was protecting. Ha ha ha.