The feed-thru terminators are 50 ohm resistors between the signal and shield. If you measure its resistance (using an ohmmeter) while disconnected, it will read 50 ohm.
An attenuator will have an attenuation value associated with it (such as 3 dB, 20 dB, etc). Its resistance will be larger than 50 ohms when measured while disconnected, though the larger value attenuators will approach 50 ohms. They are usually formed by a pi-network or a tee-network of resistors.
The divide-by-two thing is likely due to the signal source being a 50 ohm source. Once you connect the termination, a voltage-divider is formed between 50 ohm and 50 ohm, which reduces the voltage by half.