If you slide the hammer all the way back into the housing with the end cap off, you can tell by the amount of space inside the assembly. The silk is longer than the regular rod and so it takes up more space in the back end. Instead of like a 1/4" deep cylindrical space, you have an 1/8" or so.
I made an Excel spreadsheet with the grain to gram conversion for figuring out the weights. Here is on online.
http://www.unitconversion.org/weight/grains-to-grams-conversion.html