BRad i think the only time someone would ride up bikes would be pre 70 to run in the pre 75 age, there is so little differance in those era's from the public vantage point it will look the same, in that class. its also about the Rider, not the bike.
I like you make more bikes thank can ride, although most never run propally as i spread myself too thin, just because i think a bike needs to be in its pigeon hole. I dont think thisis going to be a problem, but it was more about pre 75 options for those with one bike. I cant see any justification for someone taking a pre 75 and riding it out in evo and so on, as such i thnk the peer group pressure would see them no front the line.
its also a safety net, in that if your pre 75 ride died and in your case if you where winning the championship, would you not bring your pre 70 to the line ? and be prity pissed off if we said sorry you cant ride that bike up ? its just a logical option if needed.
I think it will all work out as it always has, its just exploring the option as in Noel case where he want to ride a pre 70 in a pre75 age group, which i think is a perfectly valid entry for that bike. As that event is judging his skill as a rider and not that of the bike. Historically i think you would have also seen a 69 Cz or DT1 still on the line in 1971 on so its still in the era.
So brad you coming ? and bringing Joan ?