No there is no filling screw on the Mk IX bubble assembly and don't, repeat, don't break the shellac at any cost. These bubble assemblies are sealed and that is the reason why I told you they rarely leak their fluid and the large majority of them retained it even after 70 + years. I have so far restored 9 of them and had to refill a single one...only to discover afterwards that it still had enough fluid to form any size of bubble needed.
That is why I urge you to be 100% sure that you need to refill it. First of all try to form a bubble using the procedure described in the manual. Incline the bubble assembly 45° forward and move back and forth the round flat command knob to pump fluid and air in the chamber. You have to gently press tighter a little bit (with much care) when it reaches its range-end on the screwing direction.
If you are definitly sure your instrument needs a refill, here is the procedure to use.
The refilling channel is on top of the fluid cylindrical reservoir and is closed/insulated with a round blob of soldering tin on top.
1-You first have to file flat this blob of insulating soldering material.
2-Then you need to drill a very thin vertical hole/boring (max 0.5mm) in the exact center of the flattened blob in order to meet the original filling channel/bore. Be super careful no to let tiny metal shavings from your drilling fall into the reservoir...That's the main danger here.
3-With a medical syringe equipped with a thin needle fill the reservoir with a "small" quantity of xylene.
4-Acting with the flat command knob back and forth, pump fluid from reservoir to chamber and verify you are able to make a bubble and control its size from large to complete desappearance and reappearance at will.
5- When satisfied add a tiny extra fluid quantity in the reservoir with the syringe.
6-With a soldering iron seal back the filling channel/bore forming a nice round blob with soldering tin in top of it.
7-Let it cool down and paint it matt black.
I learned this from an autralian guy that had to do it on his own Mk IX and I did this only once on one of mines (the first one). It worked all right but it is risky albeit the safest way. Specially beware of any shavings or impurities forced into the circuit by the filling needle.
Don't be tempted to cut directly though the filling channel with a saw, cutting plier or any of the sort. I heard of a guy doing this and just definitly ruining a fine instrument.
Good luck and keep us posted.
N48 47.1 E02 10.5