AHA! EUREKA! ETC.
This handle and arm are part of modification kit 174457 (maybe see 5889S which I don't have).
50151S alludes to this -
"The 195271 Modification Kit provides a replacement tape guide for the 159953 Tape Guide on LRC200 Covers when the 174457 Modification Kit is not desired. (The 174457 Modification Kit provides facilities for external manual tape feed-out for self-contained Model 28 Typing Reperforators and includes a replacement tape guide)."
So there are three tape guides for the ROTR cabinet.
159953 - original - no handle
177404 - used with the 177405 handle via 177457 modification (like Eugene's photo)
195181 - replaces 159953 - looks like 177404, but mounts with adhesive
EUREKA! After following these clues I eventually found the 573-118-700 adjustment procedure (page 133 of issue 8) that shows this handle and arm - it evidently bears against the manual LTRS feed-out lever inside the cabinet. If your reperf doesn't have that internal manual feed-out lever modification, there's nothing for the external handle to actuate and you will be properly mystified as to what the heck it does.
references
Golly, following Teletype trivia down ratholes provides endless entertainment,
Cheers,