I find little information on understanding serial numbers yet so much can be found in them if understood. I welcome all comments to better understand this situation. Here are my observations from my 14 Toshiba's.
A. Serial number always in battery compartment
B. First is a number second usually is a Letter.
I am guessing these have something to do with regions or ? I have two KT-R2 one from Canada has a 8 as second identity while other unit from USA has a Z. Just a hypothesis that if you all can check and add your TOSHIBA model number with ORIGIN if possible to help solve what serial number means would be great.
C. Very interesting is that 11 out of 14 my models have third number always a 6. Thus I assume from observation that numbers after the 6 are the actual number of units and number of each unit.
D. It appears that in most models except the late 80's KT-4551 number 45,233 units all others are under 10,000. Is it possible that Toshiba did not make 100's of thousands like sony often did? If serial number of units made after first 3 digits is region based then my assumption here that Toshiba was especially in first 4 years a low production manufacturer then I could be wrong.
E. Appear if my hypothosis is right then my KT-RS1 was the # 882 unit made and KT-4067 the #1491 unit made making these additionally valuable.
NOTE PLEASE DO ADD YOUR TOSHIBA MODEL AND SERIAL AND ORIGIN IF KNOWN to this post and add your own observations. The QUEST for knowledge continues.

TOSHIBA

1. KT-R1 Stereo Recorder w tuner pack # 2Z180875
2. KT-R2 Canada, Stereo Recorder, MQts w tuner pack # 18603935
3. KT-R2 “ “ “ # 1Z600625
4. KT-S2 England, player case, Owners Manual, FM Tuner Module w/case
# 0Y605197
5. KT-S3 with FM Tuner Module # 2X602119
6. KT-4066 Digital Tuner, 5 presets, 3 band EQ, Dolby B, A/R # 6560002747
7. KT-VS1 Japan Release, AM/FM Tuner Module, Dolby B w FM/AM pack
# 28606118
8. KT-4026 Radio, 3 band EQ # 6960005200
9. RP-2036 FM Stereo Headphones, Analog # 7Y60010109
10. KT-4551 UK, Digital Tuner, 5 presets, Alarm, Dolby B, A/R, 3 band E/Q
# 12F5045233
11. RP-AF2 AM/FM Tuner Module for KT-AS1
12. RP-AF3 AM/FM Tuner Module for RT-CS1
13. KT-S1 with FM Tuner Module # 1Y609184
14. KT-RS1 Recording W AM/FM Tuner pack , stereo plug in twin mic
# 4Z60000882
15. KT-4026 Boxed used Radio, 3 band EQ # 6460001044
16. KT-4087 Recorder,Dolby B, 3 band EQ, AM/FM,VLSS # 7660001491