BSI Balance Weight Codes.

I have BSI window balances.  Through measureing the shoe with calipers, the delth ia 1 3/32" so I know the S385 series is what I need with the 19-005 top and 15-004 shoe.  The dilema is this; there are two sliding windows and for some reason, the installer mixed balance weight codes on each window.  Each window has a BSI 1940 (20in rated for 14-17 lbs) on onse side and a BSI 1990 on the other side.  The 1940 riviets the spring at the third hole from the top and the 1990 rivets the spring at the very top hole.  There is no cross reference for weight that I can see for the 1990 weight code.  I assume that with each increase of 10 on the last two digits the weight moves to the next range. Is that correct?  Stamp code starts on 1910 and ends at 1960 on your page here (https://www.swisco.com/20-inch/pd/Series-385-Channel-Balances/S385-20).  Is it correct to assume that 1960 is the last rivet hole at the top of the balance (the very top hole)?  If that is the case then the 1990 is really a 1960.  I weighed the window.  Its ~ 19 lbs so I was thinking of going with a 1950 weight code.  My only concern is, should I go with the next code up, the 1960, to ensure that the window still opens to the top of its travel?  Thanks for the help.