Tah-dah!
As long as you remember to always run the water pump, you’re in business until the new flow switch arrives.
That may be a Bad Sign™, particularly given the age of the machine, but let’s find out.
You have a beam combiner!
An extensive discussion has more info:
Two cautions:
- The ZnSe plate in the combiner is fragile and won’t withstand scrubbing
- The “weak” reflection of main laser beam from that slab carries a damaging amount of energy (to the left in that picture), so don’t get in the way
With that in mind, go through a complete beam path check.
Start by taping a piece of paper / thin cardboard over the entry to the beam combiner, use a manual pulse to put a light scorch on it, then verify it’s pretty much centered. You’ll surely need to weak the manual power / duration to scorch rather than burn.
Verify the scorch is a nice rounded spot (somewhat elongated due to the 45° angle), rather than some weird pattern. If it’s smooth and round-ish, then the two spots on the platform are likely due to beam misalignment that you’ll fix. If it’s weird, then the tube may be kaput.
Repeat for all three mirrors and, for M2 and M3, check at both ends of the axis travel.
Adjust the mirror angles / positions as needed to center the spots at each mirror. They need not be exactly on center, but if you must ask, it’s not close enough. 
Take the tube off the laser head, replace the lens with a disk of cardboard, and scorch. The mark should be pretty much centered.
Put the lens back in (rounded side up), stick some tape over the nozzle aperture, and scorch. The hole should be pretty much centered in the aperture.
Show pix so we can look over your shoulder.
If the machine is pretty much aligned already, that process can go fairly quickly. My machine was badly aligned right from the factory and, when I eventually fixed it, turned into an adventure:
The next several posts go into more detail than anybody should really care about, but now I’ll remember what I did.