I just looked for flights MSN-LAX on 7/5. During the same search, for the same MSN-DEN flight, I get two different fare class availabilities. They seem to match the DEN-LAX flight.

MSN-DEN UA3684 7/5
F5, A4, JN5, Z3, ZN3, P3, PN4, R0, RN0, IN0, I0, Y9, YN9, B9, M9, E9, U9, H9, HN9, Q9, V9, W9, S9, T8, L6, K5, G3, N1, XN0, X0
DEN-LAX UA372 7/5
See above

MSN-DEN UA3684 7/5
F6, A6, JN6, Z6, ZN6, P6, PN6, R2, RN2, IN0, I0, Y9, YN9, B9, M9, E9, U9, H9, HN9, Q9, V9, W9, S9, T9, L9, K9, G8, N8, XN6, X4
DEN-LAX UA1281 7/5
See above

I thought R availability was by flight, not by trip??

Thoughts?