Original sentence: The first, "Baby Babylon," was a horizontally-mounted device which was simply a prototype for test purposes, with a bore of 350 mm (13.8 inches), and a barrel length of 46 metres (151 feet).The second, "Big Babylon," of which a pair were planned (one to be mounted horizontally, at least initially for test purposes) was much larger
Edited sentence: The first, "Baby Babylon", was a horizontally-mounted device which was simply a prototype for test purposes, with a bore of 350 mm (13.8 inches), and a barrel length of 46 metres (151 feet).The second, "Big Babylon", of which a pair were planned (one to be mounted horizontally, at least initially for test purposes) was much larger