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Victor provided full support for their dealers. Here is Service Bulletin No. 7 which covers assembly and adjustment of the Victrola No. 4 Soundbox. The printing date is March 1927.
In Step 7: it is not mentioned, but after applying the wax (beeswax) heat the needle bar briefly with a soldering iron or heated awl to melt the wax around the joint to make it air tight.

Thanks for posting this. It is useful and not often seen. I have many #4 soundboxes.

I have a question... Between steps 2 and 3 there is something impossible, or I don't understand the instructions. This is the most difficult part. If you assembly the pivots and nuts into their cylinders or sockets, you already must have there the needle arm, otherwise it is impossible. For me step 3 goes before step 2, and the nuts can be screwed on the pivots only after the needle arm is in on its place. Because the central part of the needle arm would clash with the pivots if they are assembled with the nuts first!
Someone can clarify this?
Another thing is that when you screw the outer screws 15, you must be sure the pivots don't rotate, for this would alter the adjustment of pivots and nuts. All this is very tricky, and requires several trials to get it right.
I have to make for myself a pair of very thin needlenose pliers to be able to hold the pivot pins while screwing the screws. The play between the two pivots, the needlebar, the two nuts and the two outer screws is the hell to adjust properly, and sometimes a side play is left and to suppress this without gripping the needle arm movement is difficult. I usually adjust and fix one side first, then play with the other, usually several times until I get a proper adjustment. HMV soundboxes no4 and 5a/b have this system. At the end you have to play with very tiny adjustments of nut and screw... You'll need three hands.
I've always wondered if technicians had a special tool for this adjustment at the factory...