The model numbers don't necessarily strictly increase from one generation to the next. Some iPhone 5s's have A14xx model numbers and there are quite a few more overlaps elsewhere.
The model numbers always stay within a short range for the same model. A1586 is no doubt a new series because it is not within the same range as before. However, A1524 is within the range of Iphone 5S and 5C only. New phone series like 5S -> 5 > 4S -> 4 ,
ALWAYS have a model number jump which the gap is large enough to see it is a new version. (with the only exception for China only model , which none of us really concern about because they are China only)
Lets look at this:
Iphone 5S
A1533 or A1457 or A1530: iPhone 5s (GSM model)
A1533 or A1453: iPhone 5s (CDMA model)
A1518 or A1528 or A1530: iPhone 5s (GSM model China)
Iphone 5C
A1532 or A1507 or A1529: iPhone 5c (GSM model)
A1532 or A1456: iPhone 5c (CDMA model)
A1516 or A1526 or A1529: iPhone 5c (GSM model China)
Iphone 5
A1428: iPhone 5 (GSM model)
A1429: iPhone 5 (GSM and CDMA model)
A1442: iPhone 5 (CDMA model, China)
Iphone 4S
A1387: iPhone 4s (CDMA model)
A1387: iPhone 4s (GSM model)
A1431: iPhone 4s (GSM model China)
Iphone 4
A1349: iPhone 4 (CDMA model)
A1332: iPhone 4 (GSM model)