Fitting 15 is formed of a lightweight metal such for example aluminum, titanium or the like and includes a first annular sleeve portion 50 slidably received and bonded within the small end of each half-length 17 (FIG. 6), an intermediate solid portion 51 having a frustoconical exterior configuration tapering away from the end of half-length 17 and an annular shoulder 52 formed on frustoconical por