Yes, it does say CE 151, but on the photos themselves all it says is C-11 (which later became CE 150). A typographical error is indeed the most likely explanation.
Unless you have more evidence than what we can see in the WC document, I submit that it is no more likely than the wrong item number being on the photo. The two shirts in question are still in evidence in the archives. So, there is a way to test them again and compare them to the photos.
And then you call me a contrarian!
Check the record. The FBI only tested one shirt. In this case CE 150, the shirt Oswald was arrested in.
Mr. EISENBERG. Now, Mr. Stombaugh, I hand you a photograph which is labeled on the bottom "C 11, Commission Exhibit 150." It is a color photograph of a brownish textured shirt, long-sleeved, with a hole in the right elbow, and I ask you whether you recognize the shirt that is pictured in that photograph?
Mr. STOMBAUGH. Yes, I do.
Mr. EISENBERG. Can you see your mark anywhere on that?
Mr. STOMBAUGH. Yes, my mark is in red, initials "PMS" are in the collar of the shirt.
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Mr. EISENBERG. When did you receive this shirt that is pictured in Exhibit 673, said shirt being Commission Exhibit 150?
Mr. STOMBAUGH. I received this shirt the same day I received the blanket, which was November 23, 1963, approximately 7:30 a.m.
Mr. EISENBERG. Now, did you conduct an examination to determine the composition of this shirt?
Mr. STOMBAUGH. Yes, I did.
There is no mention of CE 151 or any other shirt in Stombaugh's testimony
The two shirts in question are still in evidence in the archives. So, there is a way to test them again and compare them to the photos. You won't be able to get near enough to either shirt to do a test and comparing them against photos is a complete waste of time. Besides, Stombaugh's testimony is clear enough.