Yes, i remember well a discussion on this. In fact, I remember some members questioning whether or not the 3448 could be described as a perpetual calendar because it does not have a leap year indicator. So now i will try to answer definitively.
The 3448 does not have a leap indicator. However, that does not mean that it does not recognise leap years. It does recognise leap years. So, when it is set by Patek Philippe at service, they work out what part of the 4yr cycle we are in and then set it correctly. Once it has been set correctly, it will recognise leap years and act as any perpetual calendar. Regrettably, it can take Patek a number of minutes during service to detemine what part of the 4yr cycle we are in, but once that has been set, it is fine.
Of course, one can work through the 4yr cycle oneself if one has the time, but it can take 15 -20 minutes to determine what year of the cycle we are in.
Does that make it clear?
One way of thinking about it is this....just because it doesn't show a leap year indicator doesn't mean it doesn't know its a leap year or not. It does.