Removing the rivets isn't the only alteration that is required on the front tank of the Backwoods kit to turn it from a generic NGG16 into the WHR's 138.
Both the first pair of Garratts which were repatriated from South Africa had the tank filling hatches replaced with traditional circular ones giving way to a rectangular opening with a pair of doors which are hinged on either side.
I don't know the reason why this was done but my guess would be that it means you have a bit more leeway in how you position the locomotive beneath a water column.
So this is what Himself has come up with using a few bits of styrene.
You can see that it lifts off for access to the rear screw which holds the tank into the frame.
One thing forgot was it now required a bracket at the bottom of the tank to hold the rear screw, before it was held down by the tank filler screw hole on the top.
With the cosmetics taken care of now Himself is going to start work on putting the second chassis together to go under this lot.