The Stop Arm (also known as the Armature) is the piece that connects to the rotor shaft and moves the rotor back and forth. This screw holds it in place. -cdp
Specifically for these models and their variants. Other models may qualify as well:
Horns
• 77
• 77N
• H105
• H175
• H176
• H177
• H178
• H179
• H180
• H181
• H188
• H189
• H192
• H200
• H275
• H276
• H277
• H278
• H279
• H280-H280ERA
• H281
• H288
• H289
• H292
• H378
• H379
• H602
• H650
• H651M
• H652M
• HNSOL100
• HNSOL200
• LB77
Trombones
• TR150 Tenor Trombone
• TR158 Tenor Trombone
• TR160 Tenor Trombone
• TR181 Bass Trombone
• TR183 Bass Trombone
• TR188 Tenor Trombone
• TR602F Tenor Trombone
• TR680 Tenor Trombone