We can (and I'd like to...) write pages on the MSN
systems of Airbus and Boeing.
Airbus MSNs 'seem' entirely logical, BUT Airbus 380 msn 141 doesn't mean that's the 141st A-380 built neither that it's built in between msn 140 and 142. Airbus has skipped a few numbers when aircraft were cancelled, and also shuffles the firing order a bit after allocating these msn's. For instance msn 91, a delayed Qantas machine, has still not been built while msn 148 has already been built.
Boeing is even more messy. When airlines first sign up for aircraft, the phase before the order is finalized (spin doctors call it anything these days like letter of intent, provisional order, don't shoot me on this) they usally already get an MSN
block of numbers allocated.
This is the reason why we nowadays still see aircraft delivered with msn's as low as 29146 (a recent Garuda 77W), because they got these numbers around 1998 when they first talked to Boeing about 777s. Same with the above mentioned American and Delta 737s, also numbers they allocated around 2000 after which the order was deferred.
So it's confusing as many Boeings with msn's around 29000, namely 737-500s and 757s, are slowly being retired already but you can't mistake Garuda's 77Ws as being old.
Indeed a recent order will more likely have a msn around 42000. Most deliveries on this moment are around 39000-41000, signed and allocated around 2-3 years ago.
Another trivia; Boeing allocated msn's 60082-60086 to 5 new 737-800s for Southwest. This might suggest they skip numbers 44000 to 60000, probably not to overlap with the McDonnell Douglas jets they inherited which had cn's around 43000-55000.
I love to read comments on this.