Small Wheel Attachments - Is there a standard bearing spacing?

[MattTheNoob]

Curious if there is a standard bearing spacing for the various small wheel attachments that are out there. I.e can you use the holder from one company and a mix&match of wheels from others? Maybe a few of you guys with SW attachments could grab a measurement from the outside to outside of the bearings for me?

 

[E.Carlson]

There are different styles. Bader and Esteem use an interior bearing. Kmg, pheer and lots of others use a different set up.