The wooden seat frames in the 100-4 are pitiful. Rather than replace them with new I have seats out of a later model that have steel frames. The problem is, I don't know what model so I don't know what covers to buy. The ones I want to use have the same seat pattern and configuration as the originals. Something from my memory bank of decades ago tells me they are out of a BN-4. How do I identify them so I don't order the wrong covers?