According to RCTS Part 8 the first 40 (or so) were built with 3500 gal tenders. So any of those would do, but a number of 4000 gallon tenders were built simultaneously with the first Halls (accounting reasons?) . If you have a brand new loco sitting outside the works and a new tender, do you; a. couple them together and send them out. Or b. Wait until a Castle comes in to do a swap.
I suspect the RCTS is too simplistic!
Oakhill