You`re right they`re SP ones like I have on mine,bit like hens teeth are they(Que someone saying "I`ve got a set in the shed").
Aka Eric, bit of a nightmare finding trick bits for these old buses now,looking back god I was lucky stumberling on the bits for mine!
Try this for some trick headers
http://217.199.188.40/xmb/viewthread.php?tid=23566