Maoz Tzur...Chashof Zeroah

I just spoke with my dear dad and asked him about his custom of not saying the sixth stanza of maoz Tzur.  He said it was in the German siddurim of his youth.  And he's confirmed that with others of his time. 

It seems that till recently it wasn't recited in most British Siddurim.  Here's an interesting piece from here:

Those brought up on the old Singer Siddur have always known there were five verses and have sung them with gusto all their lives, never imagining that once there was a sixth verse that somehow got suppressed. But that verse, reproduced at the end of this paragraph, has found its way back into more recent editions of thesiddur.

The 6th Verse:
חֲשׂוֹף זְרוֹעַ קָדְשֶׁךָ וְקָרֵב קֵץ הַיְשׁוּעָה
נְקֹם נִקְמַת עֲבָדֶיךָ מֵאֻמָּה הָרְשָׁעָה
כִּי אָרְכָה הַשָּׁעָה וְאֵין קֵץ לִימֵי הָרָעָה
דְּחֵה אַדְמוֹן בְּצֵל צַלְמוֹן הָקֵם לָנוּ רוֹעִים שִׁבְעָה

Make bare Thy holy arm, and bring near
the final salvation:
Take vengeance for Thy servants from the
wicked nation,
For the time has been prolonged, and there
is no end to the evil days.
Thrust away Admon in the shadow of Tzalmon.
Raise up seven shepherds.

It begins chasof z’ro’a kodshecha, “make bare Thy holy arm”, a plea to God to bestir Himself, throw off the yoke of Rome and the church, and bring about the messianic redemption.

Not every scholar accepts that this verse was part of the original Ma’oz Tzur. Some believe it is a later addition. But at least three reasons support the authenticity of the verse:
1. A medieval poet is highly likely to have referred to contemporary persecutions;
2. Poems of this kind usually end with a reference to the coming of Mashiach;
3. The first three words produce the acrostic chazak (“be strong!”), common in such poems.

Writing during the Middle Ages, when Christian persecution of Jews was so fierce, a Hebrew poet would have instinctively voiced his yearning for the yoke to be lifted. But any reference to the church would invite censorship, and this is probably why chasof z’ro’a was suppressed.

Mention of the church comes in the last line, not directly but by allusion, in the first four words, “Thrust awayAdmon in the shadow of Tzalmon“. Admon is from Edom(literally “red”), by which name the rabbis used to refer to Rome; Tzalmon, originally a hill near Shechem, is a reference to Christianity (tzelem is the Hebrew for “cross”). This is probably the only case in the siddur of a clear negative reference to Christianity.

The final phrase, “Raise up seven shepherds”, appears puzzling. However, Micah 5:4 indicates that in messianic times seven shepherds (seven denotes completeness or perfection) will overcome any adversary who attacks the Divine flock of Israel.