There is a Mark II version of the Beolab 3 and they were assigned new Type numbers of 687X
As I understand it, B&O improved their production process and used some new materials in the Mark II product which I assume was to get the product cost down. However, by doing this, it changed the performance of the Beolab 3, so to ensure they kept the same sound performance and frequency characteristics from the loudspeaker, the cross-over components were modified
These changed components which effected performance were:-
Other items already highlighted in the previous posts were also changed, but they did not actually effective the performance of the speaker.
In my view, if you had a Mark I version, you would not wish to upgrade it to a Mark II even if you could.