The Buchla Music Easel envelope generator has both transient and sustained modes, but even in sustained mode it cannot hold a note indefinitely and in fact decays slowly to silence. The problem appears to be common to all versions of the Music Easel from all manufacturers.
This is a small module which replaces IC3 and IC5 on the envelope generator card and adds infinite sustain capability. For more information about the problem and its solution, please see my analysis of the Music Easel envelope generator.
The module fits within the footprint of the envelope generator card, so will not interfere with any mounting solution which fits a 208. It is compatible with original Music Easels and the 208r revision 2 and 2.1. It does not physically mount in a BEMI card, or on a 208r revision 1, but should be electronically compatible if you care to devise your own mounting solution.
Fitting the module requires desoldering IC3, IC5, and any associated sockets from the envelope generator card. A set of four 7 way sockets for mounting the module is included in the sale. The sockets should be fitted to the module pins and the whole assembly then fitted to the envelope PCB. The module is shipped in an antistatic card box packed inside a jiffy bag.