So far as I can make out the chassis braiding to the top cap screen of the 1st IF valve was fitted to the later all glass models eg: models M and MX that had the Xtal fitted inside the phasing box, the earlier types had a plug-in Xtal that plugged in to the top of the phasing box, I remember taking the Xtal out on my plug-in one to give it a clean.
You can see the braiding in the HRO M topside chassis layout in this manual:
http://www.cryptomuseum.com/df/hro/files/hro-5.pdf
Lawrence.