For the first five seasons of Sesame Street (episodes 1-665), I have a list of the letters and numbers highlighted on each episode. Here is the list for Season 1 (1969-70; eps. 1-130)
(1-5) ESW/2/3 ESW/2/3 ESW/2/3 IPX/2/3 ESW/2/3
(6-10) ABX/4/5 ABX/4/5 ABX/4/5 JSW/4/5 ABX/4/5
(11-15) JOR/6/7 JOR/6/7 BMX/6/7 JOR/6/7 JOR/6/7
(16-20) DEM/8/9 DEM/8/9 DEM/8/9 DEM/8/9 HRT/8/9
(21-25) AHT/10 AHT/10 AHT/10 AHT/10 DGZ/10
(26-30) GOZ/2/3 GOZ/4/5 GOZ/6/7 CHT/8/9 GOZ/10
(31-35) GLY/2/3 CFI/2/3 CFI/2/3 CFI/2/3 CFI/2/3
(36-40) LUY/4/5 LUY/4/5 LUY/4/5 LUY/4/5 FKN/4/5
(41-45) IKN/6/7 IKN/6/7 LQY/6/7 IKN/6/7 IKN/6/7
(46-50) AQU/8/9 AQU/8/9 AQU/8/9 AQU/8/9 KNV/8/9
(51-55) EOV/10 EOV/10 EOV/10 PQU/10 EOV/10
(56-60) EVW/2/3 IPU/4/5 IPU/6/7 IPU/8/9 IPU/10
(61-120) same as 1-60
121 D X 1
122 B O 2
123 R V 3
124 C 4
125 G Z 5
126 U V W 6
127 I 7
128 E F 8
129 P R 9
130 A H 10
Unusual season; I sense a pattern here. Episode #121 appears to have broken a rule.
(1-5) ESW/2/3 ESW/2/3 ESW/2/3 IPX/2/3 ESW/2/3
(6-10) ABX/4/5 ABX/4/5 ABX/4/5 JSW/4/5 ABX/4/5
(11-15) JOR/6/7 JOR/6/7 BMX/6/7 JOR/6/7 JOR/6/7
(16-20) DEM/8/9 DEM/8/9 DEM/8/9 DEM/8/9 HRT/8/9
(21-25) AHT/10 AHT/10 AHT/10 AHT/10 DGZ/10
(26-30) GOZ/2/3 GOZ/4/5 GOZ/6/7 CHT/8/9 GOZ/10
(31-35) GLY/2/3 CFI/2/3 CFI/2/3 CFI/2/3 CFI/2/3
(36-40) LUY/4/5 LUY/4/5 LUY/4/5 LUY/4/5 FKN/4/5
(41-45) IKN/6/7 IKN/6/7 LQY/6/7 IKN/6/7 IKN/6/7
(46-50) AQU/8/9 AQU/8/9 AQU/8/9 AQU/8/9 KNV/8/9
(51-55) EOV/10 EOV/10 EOV/10 PQU/10 EOV/10
(56-60) EVW/2/3 IPU/4/5 IPU/6/7 IPU/8/9 IPU/10
(61-120) same as 1-60
121 D X 1
122 B O 2
123 R V 3
124 C 4
125 G Z 5
126 U V W 6
127 I 7
128 E F 8
129 P R 9
130 A H 10
Unusual season; I sense a pattern here. Episode #121 appears to have broken a rule.