Whatever program you used for cloning, my guess would be that it assigned the letter so that you could clearly id that partition during a possible future recovery. There is nothing wrong with a hidden partition to receive a letter. I 'unhide' my partitions all the time. I hate not to see what's on my system.
Many people fall into the trap of the hidden partitions. Since they see only one or two partitions in Computer, they think that's all they have on their system. And since they have never heard of Disk Management, they never discover the other partitions.
And nowadays the OEMs usually put 4 partitions on the systems - and all are primaries which is the ultimate nuisance. Only the partition that contains the bootmgr has to be a primary partition. All others can be logical partitions