I don't suppose you can get a photo of that, could you? The code normally looks lIke this:
A
68
That is, a letter (A, B, C, or D), centered over a two-digit number between 58 and 88.
Are your two letters in one line? Is the stamping clear, or hard to read?
I do apologise I looked again and it's B 84. The 4 was hard to see at first. Mystery solved. Although didn't know they were making Professional models in the 80's.