Rick,
The SF300 is EXACTLY the same as the SF600 except for the addition of two extra motors, the fluid change to the 1.91 sg fluid (vs the SF300 using the red .826 fluid), and possibly the electronic readout "Flow-Com" deal which was not available when the SF 300 was sold.
The orifices dimensions are the same size, the manometer is the same Durablock 246 piece, the dimensions are the same, same number of ranges, etc, etc.
By the way the SF300 bench was a very accurate bench for its time and would flow 400 cfm.
Tom V.