What's the difference between the iPhone with the bell symbol on the silent/ringer switch and those iPhones without the bell symbol? Several things seem to indicated that the phones without the bell image are an older model despite the fact that all iPhones report the same build and version in their software.
Why was the bell symbol added?
The bell symbol was added to make it easier to see the silent/ringer button. Without the bell symbol, you can't easily tell which position the switch is in when you don't see the orange dot. In other words, at a glance, you may find yourself trying to flip the switch both ways until you get it right.
Clicking the buttons on the newer phones, with the bell image, feels clean and crisp. The tactile feedback on the new phones make a big difference - you can feel the solid click. On the older versions, the sleep/wake button is flushed against the phone and you really can't feel it turn on and off.
The most notable functional difference also relates to the silent/ringer functionality. On the models without the bell symbol, when the phone is in sleep mode and you flip the silent/ringer switch, the screen does not turn on. However, on the models with the bell symbol, the screen does turn on, briefly, to give the user visual feedback that the phone has been switched from one mode to the other.
All of these differences, except for the last one, are aesthetics which would indicate that there's a clear difference in the software builds as well as the industrial design. I would imagine, during a future software update for the iPhone, that all versions would be updated so that the screen on all the models turns on, briefly, when flipping the silent/ringer switch.
Update: It seems I was wrong about the functional differences - there actually appear to be none. My mistake.