Beat 3 has an explicit crotchet rest, so I think “on the beat” means on beat 4, and the number 3 is a triplet of I’d say just 3 C#'s. There’s no flat sign, and the second note of the triplet is quite unclear, so I’m not convinced it’s a Bb. And there’s only three accents, not five.
Why did the composer/copyist have to write “on the beat” in the first place? I think they messed up the rhythm, writing in a hurry. It’s messy to erase triple beams. 'Ehh, whatever, let’s just add “on the beat” '.There’s a stray quaver rest after beat 3, that probably shouldn’t be there. And I even think it actually might be a triplet of semiquavers (16ths) instead of demisemiquavers (32nds).
As timpani often reinforce and coincide with similar rhythms in other parts, I would look in e.g. brass parts and the rest of the percussion, if there’s any. It may give the solution. But that’s probably unneeded, superfluous and redundant advice…
Not having seen the rest of the score, my guess is:
(My first use of the slice function, yay!)