That's a tough one to tell. I guess it would ultimately come down to where this guy is from since the description fits for both subspecies you mentioned. Just guessing I would say A. t. nebulosum... Do you have any more pictures or info on where you got it from?
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0.1.0 Colombian Red-tailed Boa (100% Het. for albino)
0.0.2 Western Hognose Snakes
0.2.1 Plains Garter Snakes
1.0.0 Red-sided Garter Snake
1.2.0 Barred Tiger Salamanders
0.0.1 Red-eared Slider
0.0.1 Common Snapping Turtle