The 5751 is a special quality double triode that was designed to resist the effects of shock and vibration. The triodes are high μ types similar to the 12AX7 but with gain reduced to 70.
The two triode sections end view. To resist vibration the bottom mica is doubled and there are two separate top micas.
The face view of one triode.
The thin glass tube envelope is 21 mm in diameter, and excluding the B9A base pins is 48 mm tall.
Reference: Data-sheet. Type 5751 was first introduced in 1953. See also1953 adverts.