I have a few of the 2017 Cab bars, both overdrive and fixed length. The line adjustments on both are made on the pigtails at the end of the lines. It's an easy process, loosen the larks head, move the figure 8 up to shorten or down to lengthen the line. Moving the figure 8 by 10mm will change line length by 5 mm.
Is it just me or is the new system from 2018 a step backwards? Seems if one of your Centre lines stretch, the only adjustment is either an overhand knot (25mm shorter) or a figure 8 knot (40mm shorter) or a combination of these to get as close as possible but difficult to get exact.