Approximation of second order system, by step response
Your actuator may look like a first order device if it is over-damped. However, it could still produce the 180 degrees phase shift between input and output that is synonymous with a 2nd order system. For example, examine the bode plot and step response of this overdamped RLC circuit (L = 1 henry, C = 1000 uF, R = 100 ohms): -



The red line in the top diagram is the transfer function |gain| and it certainly looks like a 1st order system but, the "giveaway" is the phase response (green). Note how it changes a full 180 degrees (whereas a 1st order system would only produce 90 degrees of shift).

The lower response is the step response and although it looks "1st order" at first glance, looks can be deceptive. The deception can mean that if this linear actuator is employed in a feedback system, and that feedback system does not properly accommodate the changes in phase angle, the system could show signs of instability such as hunting or plain ordinary hitting the end stops.

So , if you want to be sure about your actuator do a proper bode analysis that includes phase shift.

Interactive RLC low pass filter from here