When I bought my boat it came with a B&G 40 mm ID depth sensor and it did not appear to have been changed before. The B&G forward scanner that I bought had exactly that diameter and a length that was just enough to get in the perfect depth outside of the hull.
So, what I had and what I bought had exactly the same diameter. I don't know why both sensors you mention have different diameters
Jose G Venegas
Ipanema SM2K 278