I bought a couple of these adapters recently. This one is well made ( other one as well). What I can say is that the adapter raises the focal plane by about 1/4”. In my case i recently reworked a small 6” f/4 tube. The focuser needed to be replaced and it changed my focal plane. With the adapter, my eyepieces needed more inward travel to come to focus. So, I moved the mirror cell forward in the tube. All is well.
What I want to add about the adapter is this 1/4” utilizes a really nice brass band that snugs up the eyepieces to not mar them in any way. Collimation I imagine is also better.