I'm also looking into 37s that have the split domino headcode boxes. Again, these are preserved examples and have to be mainline certified and carrying either BR largo logo blue or Br Blue.
One locomotive I've got so far is Class 37/0 37109. It is preserved on the East Lancashire Railway and operated on the mainline up until 2008. After that, I presume it was kept on the railway. I do not know if it was mainline registered after that year.
Another locomotive I've got is 37025. Preserved by the Scottish 37 group, it is mainline certified and currently on hire to Colas Rail.
Again, any help would be greatly appreciated.
Many thanks,
James