(57) Left and right upper centering members (33) and lower centering members (34) are
provided at positions respectively opposing left and right second brackets (14), (15)
in an extension boom (5B). As engaging groove portions (33C) of the upper centering
members (33) are engaged with left and right upper connecting pins (16), (17), the
left and right upper connecting pins (16), (17) can be centered with respect to left
and right upper second pin insertion holes (14A), (15A), and can be smoothly inserted
into these left and right upper second pin insertion holes (14A), (15A). In addition,
as abutting groove portions (34C) of the lower centering members (34) are abutted
against left and right lower connecting pins (18), (19), the left and right lower
connecting pins (18), (19) can be centered with respect to left and right lower second
pin insertion holes (14B), (15B), and can be smoothly inserted into these left and
right lower second pin insertion holes (14B), (15B).
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