Holton, French Horn, 4th Lever Screw. See my item ID#
276328677477 for the main lever screw.
Fits:
LB77
H378
H379
77
H177
H178
H278
77N
H180
H189
H281
H279
H280-H280ERA
H288
H289
H181
H175
H105
H179
H275
H188
H176
H277
H276