Hello Nala,
historical speaking the U73 is a (two) bus module that had two settings, compressor 20dB compression stroke with 10dB of make up gain, or set to a fix limiting point with 10dB reduction.
There were a few "A" versions with adjustable ratio, etc. but mostly they were designed just for a two bus...
With proper modification you can use them for nearly everything and as a Variable Mu compressor it really sounds creamy and smooth.
A couple of different versions were made, E99F (similar to 6BJ6WA), EF89 and EF83, but all remote cutoff PENTODES.
The Flair of an 6386 remote triode is easy captured modified with a pair of EF97 (EF970).
I have no idea about the retro compressor but one is for sure, that the E99F(6BJ6) does not sound anything like the 6386 due to the nature of remote cut of pentodes.
You can run a variable mu pentode in two different settings, the standard pentode vs. sliding 2nd grid hook up. In the later case the dropping resistor is calculated that way that, so the voltage drop across will add more gain reduction when a certain point is reached, pretty much the same as an turbo booster in a diesel engine.
Even you run the standard pentode connection or triode mode, both tubes fall into two different design categories. The 6386 is based on the 2C51 a tube that is closely related to the 6BJ8... The E99F(6BJ6) is based on the EF95 so even in triode mode the cut off curve is entirely different...
Pretty much the same comparing an ECC82 (12AU7) to an ECC83 (12AX7)
If you want to learn it how the old time did it just use compression and gain reduction on the two bus, a limiter on drums and for the rest use just the natural compression of a room/ set up of the room.
Best regards,