Hi KK I have just had a look at my Micron rx's and black is towards the case edge. I have run a black marker pen on that side to remind me.
I changed my rx sockets for modern servo type connectors a couple of years ago.
I have had mine for about 25 years, they are 27 Megs FM. Still got 3 pairs of crystals but my preferred choice is my Futaba M Tx with the Micron Rx.