Hi - for sale an unused B.4 Maven doubler adapter (not the doubler, just the adapter). New costs $30 plus shipping and taxes. I'll do $20 shipped.
2XM Optical Doubler
The Maven Optical Double provides a small, lightweight solution to double the magnification of your Maven B and S Series optics. For use on all Maven B Series (excludes B.3) binoculars and S Series spotting scopes.
mavenbuilt.com