Sometime, around 1986, MAVIC decided to make a long cage version of their beautiful 801 and 851 derailleurs. The difference lies only in the cage plates. The MAVIC 803 was a MAVIC 801 with a longer cage. The MAVIC 853 was a long cage MAVIC 851. Just like the 851 and 801, the MAVIC 853 has a coachline on its cage plates and the MAVIC 803 does not.
The cage length on the production 803 and 853 was 70mm, which is an interesting number. This is the length of a cage for use with a triple chainset on a road bike. 70mm is not quite long enough to be a mountain bike derailleur. But by 1986, the mountain bike boom was approaching like a speeding freight train.
This is an interesting derailleur. Everything except the pulleys and pulley cage is typical of a MAVIC 851 (2nd style). The long pulley cage plates are prototypes of some sort - but nicely made ones. They have a very early-1980s-MAVIC look to them. I do not know whether they were made by MAVIC or by an enthusiastic user - but my sense is that they may be MAVIC's. The plates on the production MAVIC 853 (regretably) dispensed with MAVIC's signature geometric look, for a more curvaceous design.
Note also that the pulley cage plates on this derailleur are 75mm long, whereas the production MAVIC 853 had 70mm long cages. The, later, MAVIC 841, for road triple transmissions had 70mm plates, while the MAVIC 845, for mountain bikes, had 75mm plates. It could be that this prototype was exploring the possibility of an ATB variant of the 851! The pulley cage plates are nearly 4mm thick - which is plenty beefy enough for a spot of off-road action.
Some of the key features of this derailleur may be: