Couple of tips for dummies like myself.  Yes, you snap off that top and bottom part of the PCB.
Always test every single key before you build!  I had 3 diodes on my TX-65 that were not properly set.  Two of them responded to some flux and a hot iron.  The other one had to come off completely.  And yes, diodes are polarized.  And yes, these DO have markings, you just need a good amount of light to see them.  They are printed (debossed?) in the black.  You'll see two vertical lines and a "W1" if you look very carefully.  I removed the diode, added flux, reseated, and with a smidge of solder on the iron tip, I touched it to the diode and surface pad.