Few sanderianum backcrosses have been made, and no double backcrosses have been made as far as I know. The reason why is because they mostly suck. Just get a good Micheal Koopowitz with modern breeding, you'll enjoy it more.
I probably would agree with Tony, many of the sanderianum backcrosses are kinda disappointing...I have a MK X sanderianum in bloom now, 17" petal for now(still growing)... very much like a mini sanderianum with a lighter color pouch. It is not too bad overall IMO. I recommend it but If you like longer petals, just stay with pure sanderianum then(IMO, your recently purchased sanderianum look like a real thing based on the look of the leaves.)