There were some differences in the head castings I've seen - I don't know if it was differences in early to late model year production, or what. But on one of the other forums a guy found that out when he bought spark plugs and found they wouldn't fit as the heads had smaller channels where the plugs went in.
A lot of us who run on LPG use the BPR6ES plugs, but there is a different version - BCPR6ES I think which is the same plug, but with the smaller hex head on top, which fits the heads with the smaller channels around the plug. I don't know the reason behind the changing of it, but it is possible to get the same kind of plugs (at least in the NGK range) to fit either.