How to select the first and last elements in an irregularly shaped numpy array

I've been having a hard time finding an answer for this online, so I thought I'd query good ol' Stack Overflow.

I have this example:

v = np.array([
    np.array([1, 1]),
    np.array([1, 2]),
    np.array([1, 3]),
    np.array([1, 4]),
    np.array([1, 5]),
    np.array([2, 1]),
    np.array([2, 2]),
    np.array([2, 3]),
    np.array([3, 1]),
    np.array([3, 2]),
    np.array([3, 3]),
    np.array([3, 4]),
    np.array([4, 1]),
    np.array([4, 2]),
    np.array([4, 3]),
    np.array([4, 4]),
    np.array([4, 5]),
    np.array([4, 6]),
])

k = np.split(v[:, 1], np.unique(v[:, 0], return_index=True)[1][1:])

# output below

[
    np.array([1,2,3,4,5]),
    np.array([1,2,3]),
    np.array([1,2,3,4]),
    np.array([1,2,3,4,5,6])
]

My aim is to select the first and last element of each array in the output list. What I'd like to do is something like:

k = np.array(k, dtype=object)
new_k = (([:, 0], [:, -1]))

But alas, this is not possible. Maybe there is a way to rewrite the line that creates k to just have the first and last item?

Notice that I am trying to accomplish this with no list comprehension, defining functions, or loops - just "vanilla" numpy. If that's not feasible, any direction toward the next most efficient way of accomplishing this would be great.

The np.unique operation selects the first occurrence of a value. Doing that with the normal array means it selects the first of each group, and doing the same for the reversed array means it selects the last of each group.

import numpy as np

v = np.array([
    [1, 1],
    [1, 2],
    [1, 3],
    [1, 4],
    [1, 5],
    [2, 1],
    [2, 2],
    [2, 3],
    [3, 1],
    [3, 2],
    [3, 3],
    [3, 4],
    [4, 1],
    [4, 2],
    [4, 3],
    [4, 4],
    [4, 5],
    [4, 6],
])

first_values = v[np.unique(v[:, 0], return_index=True)[1], 1]
last_values = v[::-1][np.unique(v[::-1, 0], return_index=True)[1], 1]

k = np.vstack((first_values, last_values)).T

This gives the desired result,

array([[1, 5],
       [1, 3],
       [1, 4],
       [1, 6]])