The diaper rule is one of those rules that doesn't necessarily overlap with the rules for a bracket race car. I can run in a bracket race at a local NHRA track with my 125-ish MPH car without a trans shield. When I attend a LODRS event running S/ST with the same car it is required to have a trans shield.
Another place where the rules will vary is with the protective clothing requirements. My car has the unaltered firewall and floor still in place and has a 8 pt. roll bar. At a bracket race I'm only required to use a single layer fire jacket, but running S/ST I have to wear both the jacket and pants.
Hope this helps clear things up for you.
Hey Dan Moore, you're #1.
RIP Mark Graham & WB Hutto