If you are looking at a flute with a sterling silver headjoint, a sterling riser is the standard type. In a student flute that is silver plated, the riser would be made from the same material as the headjoint tube and lip plate - usually nickel silver. Sterling silver headjoints can have risers of sterling, gold, platinum, or some alloy of those materials.
For what it is worth... if you are going to buy a new headjoint, your best bet is to go with completely sterling silver. If is MUCH easier to find a completely silver headjoint.
Simply put, very few plated headjoints come with a sterling riser.
Now, if you were going for a headjoint that is handmade... that is a completely different ballgame.
No, what I'm finding is a student flute that is silver plated, yet it has the sterling silver riser and lip plate. The rest of the head joint is plated.
I have heard of a few companies making flutes like that, but not many.
I know that Miyazawa makes a flute like that, but it is an entry level handmade flute.
If you like the flute, and you have the money, [and it is in good condition of course], then I don't see any reason to avoid buying it. The only exception that that, is if it is one of the cheap junker flutes floating around.
To avoid flutes like that, just google the company's name, and if no official webpage shows up, then it is best to side step that company.
In that case, I have tried the 525 and to be honest, I was not impressed. I liked the 665 far better. Heck, I liked the 505 better. There was just something about the flutes that I tried that turned me off. I personally don't think they are worth the money.