There are some exceptions to the rules below but here's the general rundown.
TKG-2 is the very early upright DKs. These are Radar Scope boards that were converted to DK by Nintendo in the early days. The first couple thousand DKs were actually Radar Scope games that Nintendo converted into DKs. These were all in red cabinets. The PCBs have stickers with TKG-2 over the TRS radar scope PCBs. These have the ladder cheat (unless they got "corrected").
TKG-3(6) and below are the 4 board stacks that are most commonly in the early upright Red (and later light blue) DKs. These were the early DKs that were not converted Radar Scopes, but started life as DKs at the factory. These started off as red cabinets, but then Nintendo started sprinkling in blue cabinets, and then switched entirely to blue by the time the 2-board stacks came out. These PCBs may be found in early cocktail cabintes and caberets too. These PCBs also have the ladder cheat (unless "corrected" in the field).
Then you've got the TKG-3(7) 4-board stacks. These are commonly found in the cocktail machines and don't (usually) have the ladder cheat. They also commonley have socketed z80s.
The 2-board stacks came out soon after DK caught on. They're labeled TKG-4 and are in the vast majority of the upright DKs (all light blue cabs).
The TKG-2 and TKG-3(6) and below are comparitively rare. The TKG-3(7) and TKG-4 (2 board stacks) are pretty common.