385 balancer no weight stamp

Series 385 Channel Balances are indeed often seen in Milgard windows, but that really doesn't confirm what you necessarily need. The Series 385 is very closely resembles the Series 380, with the only difference being the depth of the bottom nylon shoe. See the depiction below, and make sure you know which balance Series matches best what you have. 

You then need to determine the length needed. Measure the metal channel, and do not include the nylon fittings in that measurement. Finally, you'll have to choose a stamp option. Since your balance has no stamp, you will need to weigh the window sash that these balances support. Match that exact weight with one of the weight ranges listed alongside each stamp option under the product description of any given balance's shop page. 

Comparison of the S380 and S385 balance shoes.