We are often asked what the difference is between DF40 connectors such as the DF40C-20DS-0.4V(51) and DF40J connectors such as the DF40JC-70DP-0.4V(53).
We reached out to Hirose and they replied that the DF40 is their standard part and the “J” indicates these connectors are for inspection purposes only and mating cycle is 3,000 times.
Top of the contact touches the contact of mating half connector.
This connector does not secure enough effective contact length for mass production sales.
They also have an FAQ on the subject located here: What is the difference between DF40C-**DS-0.4V(… | FAQ | HRS FAQ System