The exposed lead makes it a bit easier for the bullet to start expanding at moderate velocities. Or at least it did way back in time when this bullet was designed. It works, works well, why mess with success? BTW, when the 125 gr is driven at magnum velocities the bullet nose tends to fragment, creating secondary wound tracks. The remaining base generally weighs <50 grs.

I've occasionally wondered when this bullet came out in .38. It appears to be a reduced scale version of the Remington SJHP .44 Magnum bullet. At the time that one was introduced it had the most extensive/expensive development process in the industry up to that time. I've still got the first one of those I used on a white tail. Most impressive. IIRC it's about 7/8" OD with no weight loss.