I believe this is a rancho BW overdrive. Here’s a pic of one installed on an M-38a1. This was posted a month ago.
As a side note, I ran across a thread that suggested Rancho made an adapter plate to mount a rancho BW overdrive onto a Dana 20 tranny (‘herk’ comment 12/07/2007). I thought that was interesting. Sierra also made an overdrive for the Dana 20. Here’s an article on various overdrives from JP Magazine.
“used for a Jeep. Been sitting around need to get rid of it. Call Randy 775-266-4776 leave a message I’ll return calls in evening. Thanks”