In the last few years, In-Orbit Servicing, Assembly and Manufacturing (ISAM) has become one of the main and most discussed topics in space operations, ensuring increased flexibility, scalability and allowing mission concepts not feasible with current approaches based on ground assembly. From the point of view of Guidance, Navigation and Control (GNC), one of the main challenges of ISAM operationsis accurate control of systems whose properties change over time or are not known precisely, due to recurring operations of docking, separation, assembly and disassembly. Here, we introduce the TRACTOR-beam experimental test rig, used in rapid prototyping and testing for research in control and system identification with time-varying bending modes, inertia properties or additional effects which can be added in a modular fashion. The setup is comprised of a 1 Degree of Freedom (DoF) rotating platform on a low friction support, actuated with a reaction wheel, and it combines a central hub, a flexible appendage, and a robotic walker that moves along the appendage. The latter allows for variation of inertia and bending properties during the experiments. The setup realizes a ground surrogate experiment which captures the problems and challenges faced during ISAM, with low friction attitude dynamics, the robotic walker changing the system properties by moving on top of the flexible beam, and a space-representative set of sensors and actuators. The TRACTOR platform follows a modular architecture which supports distributed intelligence and modular expansion. A digital twin of the test setup was developed in MATLAB/Simulink, replicating the main dynamic effects, sensors and actuators. The wireless communication interfaces with MATLAB/Simulink provide a streamlined deployment of OBSW and management of the experiment, and an end-to-end validation chain enabling rapid prototyping and testing cycle. The paper provides a description of the test setup, digital twin with its SW interface, and experimental preliminary results in closed-loop, as well as system identification for different mass configurations.