The 25NC's with the longer tenders are the ones that were once Condensing engines(not NC's but 25 class). They mostly worked De Aar south to Beaufort West. When that line was dieselized the engines were pulled to operate on the remaining steam lines. This was sometime after 1975, cause I missed that line.
Anyway, they removed the fans off the tenders, and shortened the smokeboxes of the engines so the engines looked just like 25NC's, which they were now, but tender looked like a long vanderbilt-style as you suggested. Here's a picture of the "ORANGE EXPRESS" complete with headboard, racing northward from De Aar toward Kimberly in 1977. I remember we just made it to this rare overpass on the line, as she was probably doing 60-65 easily. We didn't bother trying to chase after this second shot.
Greg
PS I forgot to grab the url, so rather than lose my post, will add it separate.