You have two to choose from but the parts manual do not specify which was first or second. As a rule of thumb the early M38 had the rounded / spherical rear end and the later had the flatten round rear end which was also used on the M38A1.
1- WO# 802893, SPW# D2140A, ORD# G740-7351412 Horn only
1a- WO# 802894, xxxxxxxxxxxx, ORD# 8673338 Horn w/bracket
2- WO# 800906, AL# HX4001UT, ORD# G740-7369849 Horn only
2a- WO# 800907, xxxxxxxxxxxx, ORD# 8673329 Horn w/bracket
