Over the years I've had a whole bunch of Ruger Mark II pistols cross my bench. There was a spate of time when Ruger thought it would be easier to make the Ruger Mark II bolt, in two parts. The two part bolts are often found with a mainspring housing assembly that started out as a solid, one-piece casting, rather than the three piece laminate most often encountered. There have been some problems with the two piece Ruger Mark II bolts. Below is a two piece Ruger Mark II bolt:

You can easily see the seam between the bolt ears and the bolt body. The main concern, and catastrophe with the two-piece bolt is this:

Separation, that once happens can't be fixed. Ruger says they no longer have replacement Mark II bolts should your two-piece bolt separate. So, instead, they will replace you Mark II bolt, that has larger ear diameter, with a Mark III bolt that has smaller ear diameter.
As for me, I'd rather keep my Ruger Mark II pistols as close to being Mark II styles as possible. So, I have a preventative method that will forever keep your two-piece bolt a Ruger Mark II bolt. The fix is pretty much invisible and you will be the only one to know how and where it is. If this prevention interests you, send an email to
info@saronagunworks.com and we can discuss it.