The screen is a Visionox OLED with very high probability. The manufacturer name can be found when flashing the test firmware on the Zip, and it's obvious that it's no LCD/TFT when looking at a plain black screen. Bertrik mentioned in #rockbox IRC today that it behaves a bit unusually:
[18:21] <funman> bertrik: how is clipzip port going?
[18:24] <bertrik> funman, still waiting for the player hardware
[18:24] <bertrik> dfkt made some photos of the insides
[18:29] <bertrik> funman, I can do some more preparation. For example, the code to set the OLED brightness of the R,G,B components is apparently some complicated formula that is difficult to figure out.
[18:29] <funman> bertrik: you still have a link to the pics? i'm curious to see how it ressembles other ams
[18:29] <funman> you didn't identify the screen controller?
[18:29] <bertrik> I could just run the code on an emulator for a limited set of brightnesses and use a table
[18:30] <bertrik> funman, http://img.elektrokrishna.com/images...6576860729.jpg http://img.elektrokrishna.com/images...6245781269.jpg and http://img.elektrokrishna.com/images...5272451097.jpg
[18:31] <bertrik> I know which OLED controller is used (just 2 commands don't match the datasheet) and I think I also know which OLED display is used, but apparently there is a custom calculation to go from brightness to R,G and B currents
[18:31] <bertrik> or voltages
[18:31] <funman> at least the plastic cases are compatible
I assume the fact that "Visionox" can be read in the test firmware means that Sandisk service can check which "generation" the player is. They had different hardware in various Clip+ models too - seems they have to switch designs when they can't source enough components from a manufacturer. Maybe a future Zip will have a different screen, a different FM chip, etc.
The Clip+ comes is a couple of variants:
There are two slightly different OLED controllers, the new one no longer needs a 2-pixel offset and is different w.r.t. brightness/contrast setting. This is auto-detected in Rockbox.
The FM tuner chip can vary, either SI4702 (old) or RDA5802 (new), this is also the case on fuze v2. The FM tuner chip is auto-detected by Rockbox. The variant is visible in the HwInfo diagnostic screen in the original firmware.
The settings for the SD card controller can vary, this is also the case on fuze v2. In the HwInfo diagnostic screen in the original firmware, this can be seen as either "HERMON" (old) or "COMBO4E" (new).