Early Rhymney brake vans

[Penrhos1920]

I've got HMRS drawing 19843 which is of an early Rhymney brake van built in May 1881 by Metropolitan Railway Carriage & Wagon. It purports to be number 29. I have photos of 28 & 30 both with Met RCW builders plates on the solebars and they are all very similar. The RR Drawings book says that these 3 brakevans were replaced in 1894, 1898 & 1899. That implies that the original Met RCW vans only lasted 15 years on average! That can't be true. The later Rhymney built replacements were only scrapped by the GWR because they had wooden chassis. Otherwise they would have lasted 30 years or more. Or did the Rhymney have a duplicate list for brakevans?