I am wondering why Peco's NR-41S Southern Railway 7 plank wagon https://peco-uk.com/collections/2mm-n/products/7-plank-wagon4 is not numbered like all their other Big Four 5 & 7 plank wagons, was this missed by Peco, or were there some Southern Railway wagons that were not numbered?

It looks like an oversight to me.

Perhaps it's a pre-production model in Peco's photo?

There are images out there with no lettering at all other than SR

https://www.signalsmodels.com/coal_7_plank_sr_brown-details.aspx



On the other hand this shows a boxed production wagon as per the Peco image

The GW version (NR-41W) and the NE one (NR-41E) carry running numbers, as do the SR NR-40S, NR-45S, NR-42S and NR-10S. That would suggest it was not a SR policy but an omission on Peco's behalf

All wagons carried an identification number somewhere. The large numbers on the body was certainly the most common, but sometimes the only ID was a cast plate on the chassis. On the most recent wagons, Farish print a representation, but PECO have never done that level of detail...

It's possible PECO have copied a wagon that didn't have a painted number, otherwise, it does look like an omission...