Suppose we have 2 inertial systems. We know exactly, that atomic clocks of equal type run at 2 times the different rate on them. Without asking how we know that, assume that this is true. What can we say about these systems without any more data? Seems we can say they have relativistic velocity relative to each other. Seems we can say about their size dilation. We can find mass ratio. If we knew time it takes by either system's clock to transmit and receive signal from other, we could tell their distance. If we knew how time for signal travel changes, we could deduce if they approach, depart, and approximate angles of motion. Suppose, it take 2 years to transmit and receive signal measured by clock of 'faster' system. What else can be said without resorting to direct 'visual' observation?