Yep, R10. Forget the stuff in the manual about adding the pot / Variac. Just clip on your voltmeter to the battery board and watch the battery pack voltage while it charges and it should switch from high rate charge to float charge at around 28 ~28.5 volts on the pack. Your batts are probably rated for no more than 7.25 VDC, so 4 in series will mean they get balanced at around 28~29VDC anyway for sure, and probably less. DON'T set for 32VDC as it tells you in the original manual, or you just destroy the batts. A lot of times newer batts won't ever get to 32VDC, they just start getting warm and toasty (and ruined). You don't want that, and you find out in a hurry what "sealed" means for an overheated battery.
You'll also find out that modern batteries are slightly bigger and you probably don't need those spacer cards that were in with the old batteries.
R10 gets turned down to minimum (you hear the wiper clutch click) and up from there a couple turns - start about there and see about where it is. When I do these that's a good starting point. That's a multiturn pot in there. You have the top covers off.
The Batt charge light goes out when it switches to float mode, plus you'll see batt voltage drop to around 27~27.2 VDC or in there during float. That's correct. Don't over-do it.
To test you just let batts charge up until you know they are at full capacity, and then run on batt power for say 60~90 seconds, then power up on AC again and watch the system switch the batts on to high current cycle mode, then drop back to float when high-rate mode hits somewhere around 28 ~ 28.5 VDC.
You might have to readjust R10 as the unit gets warm, so set the covers on loose and let everything run a few hours and then recheck R10.
You test your unit at any time by dropping the AC power, let it run on battery power maybe 5 minutes or so, then re-apply AC. You should see the Batt Charge light come on for a while then go out when batts are charged up again. For instance if you run on battery power for 5 minutes, and then re-power up on AC: The Batt light comes on for about 30 to 45 seconds or so for a good battery pack.
A battery light that flickers or stays on always, or your unit doesn't run on battery power for at least 12 hrs means your pack or charger board is broken.
Once you get new batts in, replace 3 main power supply / regulator 'lytic caps and adjust the battery charger, button it up for another 5 or 7 years. Don't take any measures until she's all dressed up again and all screws in place, and wait a week or two after any cold power cycle. You can also check the other pot settings, but usually those are OK anyway. That's about all you do.
On older units they almost always will have drifted "up". Either leave it be and know the current voltage when you send it in for cal (comes in handy with a KVD when you have it slightly high, that means you can hit 10V on the output of KVD) or make an adjustment at cal time. Wait at least several days or so in between touching that cal pot to see where she settles in. We never worry about touching the 1.018 or 1V outputs - that's what your 752 is for. You can do those extra 732 outputs for general entertainment but a 752 on that 10V output is very, very accurate & stable for a 1V source.
Once you get it to where you want, hit that reset hole with a wire to LO to light In Cal lamp SCR circuit. That just serves to tell you if the unit went cold - out of power. Tag the front of the unit with exact output voltage / date and she's ready for service.
If you really have to there is the coarse setting board on top of the oven box, but if you have to tweak that there might be something else wrong. We've had units from the 70's that never have needed the coarse setting adjusted - from the factory they usually have the 40ppm option jumper-ed, but sometimes some other combo. The cal pot on the front should give you about 8~10ppm adjust range, more or less.
Now you wait several months and see if you have a drifty unit or not. It takes quite a while to know the full story, be patient. If you got the unit off eBay and it sat cold for a long time - it can be months (or never) before it stops wandering around. Sometimes they get good again though - you just have to wait and see.
The worst would be if a 732a sat in a cold (and hot) warehouse for years, so be very careful of "equipment dealers" on eBay.