Bob, I agree with you and I didn't specifically go out to buy a field coil for desire of originality but it was actually the cheapest option. The field coil was very cheap (1/3 price of a new starter) and I didn't have to acquire numerous old cores in the hope of finding something that works. I think because no body repairs starters, replacement parts are cheap, which is an oddity in restoring Mustangs.
I'm no expert but the reason why most field coils fail is that the insulation breaks down over time and this is just a resin that can be re-applied. I might try it one day out of interest.