I think usually they label the shafts with what they are, and it's usually located under the grip area. But this means that it would require you removing the grip just to find out.

Since this was a custom build, I would suggest to reach back out to Modern Golf to verify it. As I'm sure they (might have) fitted you and built the clubs in house, so they should be able to tell you. How close are you to Modern Golf? If you are close enough to swing by to have them verify, they would most likely do all the checking and if they need to remove a grip or anything done, it would be at their expense and not yours.

It could be just the wrong shaft labels, but again, doesn't hurt to double check with Modern Golf, as it should be able to check on that for you.

Let us know what you find out and hopefully it's just a wrong shaft label.