Need 19B replacement balances

I need some 1/2" Wide Channel Balances series 350 i think, so i am sending some photos just to be sure. On the metal channel that is exactly 20" long measured, there is "19B" inscription. If i refer to the 350 series it should be 19xx option number 10 20 30 or 40 is that number the spring strength ? What i need is for the exterior thermos window. I need Top Sash Guide, Howmet 18-018, some Take Out Clip 16-002,

Thank you Paul, yes indeed it seems to be the 350 series but it is really stamped 19B, B not 20. So i wonder if you know the right code for the exterior windows that are more heavy and need a stronger spring. I the 2 last digit the force that can hold the windows weight?

Thanks Mike, i was a little bit confuse ...
19B balance is for the interior window and you confirmed me that it is the 1920 even if the channel is really 20 inchs long.

What stamp do I choose for my exterior window balances? They are marked 19D (length is 20"). Is it 1940? I just need to know the correct stamp so I can ordert he balances and repair parts I need.

Thanks in advance.