The flat top replacement top listed above would have been made in the 1940's.  The oldest of the monitor tops are the DR series (1927-1929).  These have the open coils and the tall vertical control. The D series (1930-1932) is similar, but has the short control. the CA series 1933-1934 is the odd beast.  It uses Methyl Formate as refrigerant.  The 1933 form A looks like a soup can, the 1934 form B looks like the CK series produced next.  The CK series 1935-1938 is the most well known of the bunch.

GE was very thrifty in making monitor tops.  It's difficult to date many of them, because many of them have hardware of different vintages on them.  The hinges on the above are likely 1929-30, the latches look to be 1930-32.  The larger monitor top cabinets always looked dated.  I was told by Mike Arnold that GE sold all they had period.  It's entirely possible that an older cabinet was sold with a newer top.

That being said, the cabinet above likely had a D series top on it, a D35.  The larger DR and D tops are the most the most difficult to keep running.  You have to keep in mind that this was in the first five years of GE refrigeration.  If you look in the GE manual for the CK series, it shows that they went from to R12 for one model and right back to SO2.  I asked Mike why.  He said that a monitor top runs louder on R12 than SO2 and customers complained.  I have never found a monitor top with that suffix letter.

The FEA flat top units were sold after 1942 to replace failed older tops.  The last of the round CK series tops were made in 1941 and 42, depending on their size.  The FEA's  work wonderfully, but lack style.  Most of these have a dark blue porcelain evaporator that was used during the war instead of stainless steel

GE did advertise a "flat top" unit as early as 1935, but that was a flat top refrigerator.  The round "monitor top" compressor was below the refrigeration compartment.

John,

That's great to know.  I was reading a service manual designed for service on the CK series monitor tops.  The last date mentioned was 1942.  You could see they were experimenting with R12 in 1938 or 39.  I believe R12 was invented by Frigidaire.  It makes sense that GE wouldn't want to work with it.

I know there's a manual that covers each series of these.  I would bet there's a manual for the FEA, the flat top.

The condenser coil on a monitor top are the exposed tubing on the oldest ones or the circular bands on the newer, CA or CK ones.  The condenser is large enough that it doesn't need a fan to function.

The control knobs changed by the years.  The DR controls are the tall control, the D went to a short square control, the CA form A has the control mounted on the "throat" of the unit.  The CA form B and later CK controls were in the center of the condenser band.

The mindset back then was to reuse what you had and replace only what was needed.  I would assume that buying a replacement top for your existing cabinet was cheaper than buying a new refrigerator.  It might be that the flat top units got used in the used appliance market or simply during WWII when you were thankful that you had a refrigerator.  My grandma has a picture from about 1953-55 showing a mid 1930's monitor top in her apartment.  I am sure she was thrilled to not call the ice man!

Phil,

That's also a flat top on a late 20's-early 30's cabinet.  Someone has the cover on wrong.