Could be Compression ratio differences. Early 1275s (8.8) used the Champion N9Ys while later ones (8.0) used the N12Ys. N12's are colder than N9's. Probably the same arrangement for the 1500s. A while back, I made recommendations for spark plug heat ranges based upon compression ratio. From a NGK part number perspective, I started with the BP6ES and worked up to the BP9ES (actually going colder) based upon compression ratios from 8.0 to 1 to 14.0 to 1.
8/9 to 1 gets 6's
10/11 to 1 gets 7's
12 gets 8's
13 to 14.5 gets 9's
14.5 and above get's 10's (Note: Usually BP10EV's [Exotic metals & Fine Wire] are usually used at this high of a compression ratio)
Colder plugs justs means that the burn heat is "wicked" away quicker by the spark plug metal construction. Usually means less ceramic insulator.
Also, the sparkplug design changes depending upon application BP are "projected nose" plugs. B is regular nose. BPR would be a projected nose resistor plug.
Run as "warm" a plug as you can safely accomodate. Start colder and work hotter. This is the conservative approach and avoids bad things happening. Warmer plugs are good for stop and go driving, while colder plugs are better to highway type crusing and sport applications.
HTH
Mike Miller