The item ID's are an auto incrementing field in the database. As they added items during development each item added is automatically given the next available ID number.
Since instances like dire maul or BWL have a much longer development time than say adding items to existing instances, the ID's aren't going to match up perfectly. For example, while patch 1.3 was live, they could have been working on BWL already and creating items for it but BWL was not released until 1.6. Later on, when 1.4 is live, they add items to existing bosses in MC and these items go live in patch 1.5. As a result the items added to MC in 1.5 could potentially have higher ID's than items added with BWL in patch 1.6.
This means you can't just look at 2 items ID's and say "this was patch 1.5 and that was patch 1.6"; however, you can look at all of the item ID's from a particular boss, say Magmadar (set pieces ignored to keep the list shorter):
http://db.vanillagaming.org/?item=17065
http://db.vanillagaming.org/?item=17069
http://db.vanillagaming.org/?item=17073
http://db.vanillagaming.org/?item=18203
http://db.vanillagaming.org/?item=18820
http://db.vanillagaming.org/?item=18821
http://db.vanillagaming.org/?item=18822
http://db.vanillagaming.org/?item=18823
http://db.vanillagaming.org/?item=18824
http://db.vanillagaming.org/?item=18829
http://db.vanillagaming.org/?item=18861
http://db.vanillagaming.org/?item=19136
http://db.vanillagaming.org/?item=19142
http://db.vanillagaming.org/?item=19143
http://db.vanillagaming.org/?item=19144
Does anyone else see a pattern forming here?
The items starting with 17xxx were likely on the loot table at release.
From the official patch notes, we know that items were added to MC bosses in 1.4:
"Onyxia, Azuregos, Lord Kazzak, and all Molten Core boss creatures have all received some new items on their loot tables."
We also know that additional items were added in patch 1.5:
"Additional new raid items have been added to the treasure lists for Azuregos, Lord Kazzak, and bosses in Molten Core."
So, we have 3 distinct groupings of item ID's (which means that these items were developed around the same time), items were added to MC bosses twice, once in patch 1.4 and once in patch 1.5. The truth is pretty obvious at this point but just to make sure we can check a few sites like thottbot and allakhazam on web archive to see if we see anything that disproves the theory. Note that if an item first appears later than we would expect that isn't conclusive, it's possible that the item page wasn't cataloged by web archive until later and no comments were posted.
As I'm sure most of you remember, back in vanilla the game client updates were usually available a few days before the patch went live on the server. Sites like thottbot and allakhazam mined the data files that came with the client to generate lists of items BEFORE the patch went live. This is why many items have comments on them 2-3 days before the patch date.