Okay, I am finishing the Magnavox phonograph,
and happened to see two of these bubbled in the amp.
Are they ceramic capacitors? Resistors?
The only numbers marked are:
an R at the top with three lines on the left of the R,
.005
M
Z5u
Anyone know what I should replace these with?
Thanks!

Magnavox300

That looks to be a ceramic capacitor. It's small enough though where I'd just go with a silver mica of the same or higher voltage rating just to be safe.

They are disc ceramic capacitors, .005 microfarad with 20% tolerance and sloppy temperature coefficient. Probably just used as bypass, and nearly any ceramic will work fine. Probably best to stick with 500V rating. They should cost a dime or so.

Having said that, I suspect the ones you have are still good.

Ditto, ceramics seem to last forever. That bubble might even be how the original coating ended up, not a trouble spot.