Game Boy Types

Different builds
Depending when and where it was manufactured, the Game Boy Classic hardware (DMG-01) can vary greatly from one device to another. According to the Game Boy Hardware Database, the traceable significant changes are for:
 * the mainboard
 * the CPU
 * the VRAM
 * the WRAM
 * the LCD board
 * the power board
 * the jack board

Why does it matter?
For playing this does not matter very much, however for audio applications and modding, there can be some issues with earlier models. The three things that can be an issue are:
 * Early GB models have high pitch and sample playback issues with LSDJ. Some of these issues have/can be mitigated with some LSDJ updates, but not totally.
 * Late GB models have their main chips under epoxy blobs, which means that these chips will not be serviceable nor can all their pins can be accessed
 * Early/mid GB models have a crappy voltage regulator, which means that they will tend to overheat, specially when they need to also provide power to demanding computation all the while maintaining a backlight mod for the LCD screen. Early voltage regulator are also said to introduce extra background noise. In practice the two generation voltage dividers have their own variation, and it is said that the latest variant(s) of the second generation is the most efficient.

How to check
There are 8 mainboard revisions and the consensus in the ChipMusic.org community is that any DMG-01 starting from DMG-CPU-4 is fine for LSDJ. But that does not mean it's fine for modding or using some flash cards. The older the model is, the most likely the voltage regulator will be of the second, better, type. To be sure, unless you open the GB you will not know exactly. However there is a small trick to know from the outside a bit more about what you're likely to get in the inside. If you remove the battery holder and check through the small hole in the case, you will see the mainboard revision number and that should tell you a few things.
 * DMG-CPU-01: CPU variant unsuitable, no blobs, first gen voltage regulator variant ❌
 * DMG-CPU-02: CPU variant unsuitable, no blobs, first gen voltage regulator variant ❌
 * DMG-CPU-03: CPU variant unsuitable, no blobs, first gen voltage regulator variant ❌
 * DMG-CPU-04: CPU variant suitable, no blobs, first gen voltage regulator variant ✅
 * DMG-CPU-05: CPU variant suitable, no blobs, first gen voltage regulator variant ✅
 * DMG-CPU-06: CPU variant suitable, no blobs, first or second voltage regulator variant ✅
 * DMG-CPU-07: CPU variant suitable, blobs, first or second gen voltage regulator variant ✅
 * DMG-CPU-08: CPU variant suitable, blobs, second gen voltage regulator variant ✅

No perfect Game Boy
Before you go full OCD on trying to find the perfect GB, just keep in mind that there is no such things as a perfect GB:
 * Even with the right mainboard, etc, you can have a GB that has some faulty/aging components
 * The naming convention can be confusing for the CPU because the mainboard revision contains the word CPU as well. In practice the same mainboard DMG-CPU-0* version can come with different DMG CPU * processor variants
 * GB are usually quite resilient, you can mix and match different "broken/for parts" models to have a fully working one
 * There are several mods that can be done to improve the shortcoming of the some/all GB issues
 * New parts are regularly being developed to improve/change the screen, replace the original voltage regulators with brand new modern ones, etc. So we can easily follow this universal rule of GB modding: given enough time, any perfectly modded GB will become outmatched again :)