I am not sure a "standard" existed.

Earlier Nitro DR's SEEMED to have longer barrels
than later (newer) ones.

Certain makers (retailers) seemed to make more shoter ones than longer barreled ones - Army and Navy being one retailer who most guns I've seen have long barrels.

I think the type of action the gun was built on + calibre may have partly determined the barrel length due to having to get the "balance" right.

Re" most 470's I've seen are 24", have a look at some more, I have seen plenty with longer - 26".

Re "I've seen a lot of 450/400's with 28" barrels"
have a look at Jeffrey's (made by Leonard) and a whole
heap of others - Rodda comes to mind. Early 450/400's, yes, 28" but they got shorter.

This is just my opinion from observations. I would like to hear others as it is an interesting topic.