Most, probably all Mk IV tenders were delivered ex factory rather than being later conversions of existing tanks. The sponsons were not armoured. Was the rest of the tank fully armoured or were tenders built using mild steel (or boilerplate) - anyone know for sure? When were Mk IV tenders first delivered?
An analysis of serial numbers has now enabled me to tentatively (though pretty confidently) identify the 205 factory built Mark IV Tenders.
This also shows that there are quite a number of Mark IV Tenders that have numbers outside these 205. I can also find tanks that are recorded in 1917 as fighting Mark IVs (i.e. Males or Females) but which crop up as Tenders in 1918. So I am sure that not all Mark IV Tenders were delivered ex factory and that many were actually conversions of fighting tanks.
I am not clear whether the sponsons were unarmoured in all cases, and I do not know for sure whether the remainder of the hull of factory built Tenders was armoured or not.
I have H51 at Cambrai as being a tank named Hiawatha. I don't know of H53. The manufacturer's numbers of H Battalion tanks at Cambrai is a bit of a problem area I'm afraid.
