It's a really atmospheric photo.
Keep in mind that the event may not have been "dirt track" in the sense that we usually think of it. Road races in the 1920s were held on unsealed public roads, as were reliability trials, hill climbs, fuel consumption competitions and so on. Here in South Australia beach racing was very popular in the 1920s - the course was a stretch of beach with a 44 gallon drum at each end as a turning marker! Then there were flag races and gymkhanas of all kinds.
As Eddie has often told us, a 2 3/4 was a pretty quick little bike.
Cheers
Leon