I copied this from the Drake list server in early April. Consider the following quoted text. I left Tom's name in here simply because he should get credit for it and the message was sent publicly.

    I have two different TR-4's. These are plain TR-4's; not TR-4C's or later models. I was surprised to find a number of differences in these two radios. The early TR-4 has many characteristics of the TR-3, while the later TR-4 shares features with the TR-4C.

    By comparing your TR-4's serial number with mine and your radio's features with the list below, we might be able to figure out when each feature was introduced in the TR-4.

    The serial number of my early TR-4 is 1965. I noticed that the rear view picture of the radio in the TR-4 manual (Fig. 7, p. 17), shows a TR-4 with a serial number of 16050. My early TR-4 has these characteristics that differentiate it from the later TR-4:

• Has a TR-3 style main tuning knob
• The 9 mc sideband filters are sealed inside a soldered box. Bill Frost of Drake calls this the "soup can" model and says that the filters inside are four pole filters.
• The tubular capacitors in the radio are white, larger, and made by CD.
• The "Plate" and "Load" markings on the front panel are printed in silver letters on a gray background.
• The red dial indicator is the full vertical height of the frequency plastic window.
• There is no noise blanker knob on the lower right front panel.
• There is no receptacle for a 34PNB noise blanker on the chassis

    The serial number of my later TR-4 is 31985. It has these differentiating characteristics from its younger sibling:

• Has a TR-4C (or R-4C) style main tuning knob.
• Has a VFO "in use" light above the main tuning knob. The indicator is driven by a two transistor board mounted behind the noise blanker switch.
• Has an open sideband switch (versus the sealed "soup can") and two blue Tyco filters. Bill Frost says these are eight pole filters.
• The tubular capacitors are yellow, smaller, and made by CDE.
• There's a little circuit board behind V17, the 6AQ5 audio output tube. The circuit board is labeled, "TR-4 audio" and it is an audio pre-amp.
• "Plate" and "Load" markings on the front panel are printed in black letters on a gray background.
• The red dial indicator is the one third the vertical height of the frequency plastic window.
• There is a noise blanker switch on the lower right front panel.
• There is a receptacle for a 34PNB noise blanker on the chassis