I thought this would be a perfect way to show the differences between the Simplex 4207 and 4208 fire alarm panels.
First off, the Simplex 4208 came out first. I have seen a 4208 dating back to 1970 on eBay, and I have even seen 4208 diagrams dated 1969! I believe the 4207 came out in 1975, and both were discontinued in late 1978/early 1979 in favor of the 2001.
Worth of note is that the 4207 and 4208 both came in multiple sizes and configurations. They were not like the Simplex panels since the 1980s. Like many older fire alarm systems, they were often designed specifically for the system they would be installed in (this was especially the case with 4208 systems.) They both had numerous options available, and could be used as 120VAC systems or 24VDC systems.
A 4-zone 24-VDC Simplex 4208 panel from 1974. The reason it is large for a 4-zone system is due to the features that were included with it. Note the battery cabinet next to it, showing that it is a DC system. The reason the alarm indicators on the 4208 are on is because a supervisory current is passing through. This is completely normal on older 4208 systems, and the lights are actually a bit dimmer than they should be. Should a zone go into alarm, its indicator light would be brighter.
The amp meters, trickle rate and signal silence switches. Also note the older-style Simplex fire alarm logo on top. This was used from 1969-1970 until 1978.
The drill switch, zone and trouble indicators, trouble piezo, silence and reset switches.
In contrast, here is an 8-zone 24-VDC Simplex 4207 system from 1977. The 4207 cabinets like this had a double-bay look compared to the 4208 above. Also note the different-looking trouble piezo, and the placement of the indicator lights and the switches. Also, the 4207 does not have all the alarm indicator lights on like 4208s usually do (they are apparently wired differently.) The battery case is hidden, presumably in the closet next to the panel.
Close-up of the 4207 controls. On this particular model, they were able to put all the lights and switches right near each other, instead of spreading them out like on the above 4208.
Here is a similar Simplex 4207 system from 1978. Again, it is an 8-zone 24-VDC system. There is some kind of A-key switch that is unmarked. Also note that it has the newer Simplex logo on the cabinet. The later 4207 and 4208 systems made in 1978 had this.
Inside of the Simplex 4208 system at the top of the thread (the system was being worked on at the time these pics were taken.) It was a relay-based system, and presumably Simplex’s last one. Note that this particular system has two coding wheels, an optional feature on Simplex’s pre-1979 systems. (I believe one was for the horns, and the other for visual signals. The Simplex 4010 tied into this now handles the alarm signals.) It also has a relay to trip the Gamewell master box at the main entrance so it can contact the fire department when set off (the 4010 also now handles system monitoring.)
Lower half of the 4208. Note the newer Simplex relay thingies on the bottom right. I am not sure if these replaced older relays, or if they have something to do with the 4010 system.
Inside an abandoned Simplex 4207 panel manufactured in 1976 (it wasn’t actually installed until 1978, and was removed around 2000-2001.) This was also an 8-zone 24-VDC system. Unlike the older-style relay setup of the 4208, the Simplex 4207 systems were more card-based, like more modern hard-wired conventional systems such as the 2001 and 4002. Also note the early microprocessor boards, along with the coding wheel (this was also optional on 4207s.) 4207s also had voice-evacuation as an option late in the system’s life (though back then, voice evacuation was more common for large high-rises or other extremely large facilities.)
Felt like sharing to help clear up any confusion. 
