There are 4 models: 101, 103, 105, 107
Each and every model has two variants A and B.
A and B are most definitely the colors.
Every model has three qualities Good, Better and Best.
This is very likely space, either 8,16,32 or 16,32,64 because every model has those three variants.
The tricky part is this:
There is two groups, Standard and BTR. 101,103 are BTR, 105, 107 are Standard.
101 and 103 do not exist as Standard, and 105 and 107 not as BTR, so it cannot be space and it also cannot be provider. Provider is further unlikely because BTR most likely stands for "Better" and it would make no sense to distinguish providers in quality. It may likely be connectivity, WiFi vs LTE.
But what distinguishes those two WiFi models and the two LTE-Models from each other? Can you guys think of anything? I cannot.
If it is the panel size, that would mean that there's no distinction between different providers anymore and that the large iPad will also come in two colors...