One of the first Watchtowers has Jephtah as actually making a burnt offering (actually killing his daughter), and that the sacrifice being "symbolic" was only a "possibility" but not really supported by the Bible. Here's the quote:
September 1, 1901, p. 14
WHAT WAS JEPHTHAH'S VOW?
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Question.--What attitude are we to assume toward
the account of Jephthah's reckless vow which brought
death to his daughter? Is there any redeeming feature
in the incident?
Answer.--We are to accept the scriptural statement
that Jephthah was amongst the faithful--acceptable to
God. As such he must also be acceptable to us. In
respect to his offering his daughter in sacrifice our
conclusion must be that the divine arrangement then
and now differs materially. We may say, however,
that as Abraham was about to offer his son Isaac, not
willingly, but through obedience to what he understood
to be the divine will, so did Jephthah with his
daughter; and he was not hindered by an angel from
the Lord. I do not know if the lamentation has any
significance.
Answer.--A totally different view of this matter is
possible, and we merely suggest it; namely, that the
vow was one of full devotion to the Lord--one of
chastity and sanctity--seclusion from society, deadness
to the world as a priestess. The daughter's request
for time for lamentation, and the subsequent
annual celebration by the virgins, would agree well
with this view. The chief objection to this view is
the statement respecting "a burnt offering," and this
seems almost insurmountable.