My good fellow, what you have is INDEED an Underwood 150.  But I am not sure it is a '55 model.  I have an identical machine that was made in 1956.  It's pica type.  What is the serial number on your machine?  I can probably tell without looking it up.  If it begins with 78 or 79, it's more than likely a '56 model.  That brass script on the front (which is notorious for tarnishing), was used into 1957 before Underwood went to "Golden Touch," machines--shaped pretty much the same way as the 150, only with a few minor internal mechanical differences.  What you'll want to pay the most attention to is the underside of the noise deadening felt on the insides of the panels--particularly on the front panel.  Remove them and get off the rust that seems to be attracted to whatever paste they used to glue them in with.  Outside of that, you have one of Underwood's finest typewriters--a very fine, smoothly operating typewriter with an even smoother carriage.  This machine, once in tune, will outlast you and your grandkids.  Again, tell me what the serial number it is, but it looks all in the world like a '56.

Forgot again--as for that sticky G, just a drop of tranny fluid in that particular slot should free it, and if that doesn't do it, try pulling up the type bar to see what side of the type bar guide it tends to go to.  Ever so gently, push it a little toward the other way and try each time to see if it still sticks.  You don't want to bend it too far because if you do, it will still stick, only on the other side of the type bar guide.  Just little nudges.  Oh, and by the way, I saw the picture and it looks like you have a pica type machine too.  I saw the scale centered on 50.  If it were elite type, the center would read 60.  Does it have a special typeface?  Or does it have the usual with the DP on each type bar (for Distinct Pica)?  Let me know.

Well bless my soul.  It must be a late 1955 model.  I thought they only came in taupe with grey keys with navy blue surround keys (shift, backspace, margin release, tab clear, tab set, shift locks).  They also had an aluminum badge that read: "Speeds the World's Business."  I have a '55 model 150 elite type that fits this description.  When I get set up, I'll post some pictures.  Right now, one of those digital type cameras is a bit too rich for my blood.

Very nice machine!