What I did:
I fited the resuts of IP's resonant frequency to theoretical value.

The equation used for fitting was atttached.
f is IP's resonant frequency.
k_e is spring constant of IP's flex joint .
k is s anti spring constant.
g is gravity constant.
l is length of IP.
m_0 is mass of IP.
M is weight on IP.

From fitting, m_0 is 1.08 kg, k_e is 131 N/m, l is 2.48 m.
 

The results and fitting picture was as follows.
X-axis is weight on IP top, y-axis is IP's resonant frequency.