They’re color coded. Nust buy 6 (3 pairs) of the same color code.
No only the pairs need to be of the same color and installed in the proper slots as pairs.
You can even use different varieties of 6L6 and different colors in the same amp.
For example;
green 445, blue 440, blue 445--------------blue 445,blue 440, green 445
The green 445 are your outer slots, the blue 445 are the innermost slots, and those 440 are the slots in-between (tweeners I call them),
The "tweeners" are the slots that run if you switch a Multi-Watt Triple down to 50w.
Like they always say about Mesas,,,,,,,"do not confuse the rectifier slots on the far right (as viewed from the back) with the power-tube slots!"