[TA Forbes]

The 37 and 76 can be substituted for each other. Personally, I prefer the correct tube in place, but fyi...

I don't think that the osc issue has to do with the tube, but agree putting the correct tube there is the way to go. I would recommend you continue to replace the rest of the wax/paper caps if not done so already.

Have you made sure that you are aligning on appropriate peak and not a harmonic?

Have you tried adjusting the distance of the shadowmeter bulb filament from the aperture of the meter? This controls the width of the shadow.

Moving the bulb filament from left to right controls centering.

Be careful not to short the hot end of the bulb socket to the mount, it is, as you shall see, easy to do.