Mike W
Western Thunderer
The last LNWR horseboxes to have top hinged upper doors were the first 19ft 6in examples in 1880. The GA drawing has survived but in dreadful condition, so attached is a re-draw of it done by my brother Bob Williams for the LNWR Society Journal many years ago. In this case dropping the lower doors automatically opens the top doors, but your handles are on the top doors, so the mechanism may be different. The LNWR is of course a double ended horebox, so the mechanism is entirely inside. I don't know on the single ended types whether it was inside or outside, but the LBSC drawing shows none of it, so maybe it was all hidden, in which case this is a bit irrelevant to you. But hopefully of interest anyway.
Mike
Mike