I know what you are saying about the engine nacelles.. I will try to get you an answer ASAP. I have noticed it too but didnt give it much thought as I dont see the Tu-154B very much these days.
The main structural difference between the Tu-154A and B series (B is a series, there were variants within 'B') is the wing.
Aeroflot put the original Tu-154 into service before the static fatigue tests were complete. The initial test results were good enough to allow it to enter service, but simultaneously the tests continued in the background. At the time there was a big need to replace types like the Il-18, An-10 and Tu-104/124 quickly.
The results from the fatigue tests came back from Novosibirsk and it was clear that the wings were never going to make the number of cycles required (about 20,000 cycles / 30,000 hours in those days).
Here's where is gets a bit difficult to give you a clean answer.
The first 120 were recalled (minus the ones which were destruction tested) to have the new wings fitted. Some of these were not yet delivered.
The production list shows that only the first 40-50 were 'A' models, since while the aircraft were back in the factory the opportunity was taken to upgrade them to B models.
Therefore all A's and some B's were re-winged. A lot of the Tu-154A's were upgraded to B standard according to the production lists.
The only foreign 'non-Aeroflot' customer to operate the Tu-154A was Balkan Bulgarian Airlines (LZ-BTA, BTB, BTC). I believe these were all upgraded at some time. Some production lists disagree with that point.
The only external difference is a small 'point' with the tail. See the following pictures:
Can you see a difference with these?
I know it's a long answer, but I've got nothing else to do while I wait for paperwork from the friggin CAA.