The Tektronix SG504 is a levelled sinewave generator plug-in for the TM500 system.
The SG504 is usually found on Ebay without the output head (015-0282-00). David Partridge did an excellent teardown and reverse engineering of the output head and sells kits to make new heads.
It's fairly easy to use that schematic to make a new level head that fits in a standard BNC Pomona box since its function is really just a 250 MHz to 1050 MHz temperature compensated power meter with 50 Ω feed-thru. See the pictures for an example.
As of April 2016, Ebay seller "ancelb" is selling SG504 leveling (output) heads that are specified to perform much better than the original Tektronix heads. See photo below.
There is an open source project to replace the output head at hackaday.io.