My question is what was used in the cars when they came out of the factory? Was it all hard colored vacuum lines, with the black just for connectors?

I have a 76 450sl, and the vacuum lines are a jumble of the original hard colored line, with some soft black rubber lines mixed in. Sometimes the connection from the rear firewall to a final connection jumps back and forth, from hard line / to soft / back to hard - all in a single line connection.
This doesn't seem good.

I ask because the two lines actually have different inside diameters. The black is bigger, and as I learn how finicky the vacuum systems on these cars are I wonder about the relationship between diameter and pressure,
Shouldn't the smaller hard lines help keep pressure higher?

I'm going through my car and checking/replacing all suspect vacuum points (I'm finding a lot of cracks) but I wonder what the ideal replacement is, hard line or soft.
Thanks.