So we all know that Covenant weapons and vehicles have number designations, such as Type-25, Type-32, etc, which correspond to the year when the UNSC categorized it. (and not the year the weapon was developed or the year it was first encountered) Exceptions include melee weapons, such as the Energy Sword and the Gravity Hammer which are respectively designated Type-1 and Type-2. I wonder why it works differently. (same question for the Promethean weapons)
But the Type-51 Covenant Carbine has always been a bit out of place. It was used on Harvest, so it would have been strange for the UNSC not to categorize it early in the war. Of course, it’s always possible that they didn’t manage to. For example, it’s the case for the Type-56 Lich, which only received a designation in 2556, after the war was over, despite the fact that it was used during the conflict. (It’s said that it was a bit rare, and most of the time the humans were annihilated anyway)
But I’ve just realized that there could be more than one Carbine. I mean, it’s something that’s been introduced in Halo 4 with the Beam Rifle: there are a Type-27 (H4/H5) and a Type-50 (H2/H3) (and maybe more!). Same thing for more recent Halo 5 weapons (a new Plasma Pistol, a new Needler, a new Fuel Rod Gun…)
So, there are many variants of the same weapon. When differences are minor, the designation receives a letter. (e.g. Type-54D) Interesting enough, the Carbines through Halo 3 to Halo 4 look different, but they are still called “Type-51”. So, I suspect they might be differentiated by letters, although we don’t have these designations yet. (e.g. Type-51A, Type-51B, etc)
Now back to our year, 2551… It’s clear the Carbine wasn’t introduced lately into the Human-Covenant War. So here’s my theory: it may be that the Carbines we’ve seen in early stages of the war (during the Harvest Campaign, etc) were not Type-51 Carbines. It’s like the Beam Rifle: there’s an old one that was categorized early on, then it kind of disappeared and was replaced by a more advanced one. Except that the old Beam Rifle from the 2520s resurfaced in the post-war era, because of the Covenant’s lack of resources, whereas the old Carbine didn’t return because the Type-51 was still available and probably more effective. Then it was kind of pushed aside by a new model, the Type-57 Carbine (Halo 5), which proves there are indeed many Carbines too.
So to put it in a nutshell: the Type-51 Carbine has this “late” designation despite its early appearance during the war, because perhaps there was a Carbine predecessor, for example with a name like “Type-28 Carbine” or whatever early year it is related to. I don’t see any other possibilities. (unless the UNSC really did not manage to study it properly until late into the war but, well, I don’t think so)
Bonus: Sali 'Nyon’s Sangheili followers used Needle Rifles during the Absolute Record arc late in Halo: Escalation. These weapons belonged to Jul 'Mdama’s Covenant faction. Were these the well-known Type-31 Needle Rifle used during the Covenant War at least up until the Fall of Reach… Or could those be something else? Like, say, a hypothetical Type-55 Needle Rifle? That’s some food for thought.