The 840 derailleurs and siblings have Dura-Ace 7400 geometry. There is a guide on the derailleur that can be orientated in two ways. One is for 6 speed or essentially friction and the other is for indexed. Shimano 7400 downtube shifters and brifters will work with the 84x series.
Here is a setup I have with 7400 brifters, Mavic 841 rear derailleur, and 571/2 rear hub: