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Just to add a different slant on the issue. The Udasi themselves do not consider themselves Sikhs in every instance. It really depends on the order within the samparda.
Having said that, some Udasi certainly did consider themselves Sikhs all the way through to 1925 and the installation of the Sikh Rehat Maryada.
Between 1883 and 1924, the movement away from vedic traditions and toward plain guidance from Sri Guru Granth Sahib Ji put Udasi into conflict with the emerging Singhsabha movement. Before Singhsabha split into two camps, Amritsar and Lahore, Nihangs, who were part of the movement to drive Udasi mahants from positions of power, (and who also have their own rehat and 3 scriptures not 1) did not consider Udasi Sikhs. After the division and the formation of the post-1925 version of the SGPC, events demonstrated that Udasi would not and could never unite with either group.
Who is a Sikh? It is not so easy a task to decide that when considering among the many different sanatan orders. Difficult particularly when many of them dispute the legitimacy of the Sikh Rehat Maryada, going as far as to state that the document was a corrupting influence, imposed by the British, to separate Sikhs from their puratan heritage. Therefore using the SRM to decide who is a Sikh falls on deaf ears on the puratan side.
Do I believe that? No! I do think we are discussing the best way to walk through a mine field.
Forgive me for glossing over all of the particulars of history that bring us post 1883 to where we are today. Mention of the Udasi is guarantee to trigger a lot of passion. It is not even important to debate their Sikh identity in my opinion because they did lose a fair fight in courts of law and fell from an exalted position.
I agree with one thing. The entire story cannot be understood by reading a few Internet pages.