Yes, 112

tube***

Are you sure you meant terminal 122? I don't see one. I see a terminal 112

 In which country are you? what the 3 phase power input? 380VAC?

give more machine history 

tube***

However, E/R module is still only showing 300VDC at the dc bar, not 600V.

 There are 2 questions:

#1--it is the MUST T.9 to T.48 and 112 to enable the Hi voltage DC 600, normally machine builder will make some condition to enable, as (I think you got 2nd hand machine) do not know the sequence how to run it. --Yes you can direct jumper those to make machine running --suggest using a switch --can be on/off T9 to T48, when start up make it off, when CNC and all booting good, then turn it on.

before shut off the machine (after work) turn off that switch 1st then shut off mains

#2--I don't know why you only measured 300VDC, I never see case like that --I worked for more than 100 cases-- 400VAC should be good , can you double check you meter and post picture of the setting switches on the top of E/R module --I add a pic to show you the location--there should be 4 to 6 dip switched 

tube***

So do you think it's safe long-term to keep terminals 9 and 48 jumped on my E/R module?