Dotted tremolos, marked "12"

In bars 153 and 154, I have dotted tremolos with a 12 over them. I don't know how to recreate them. Dotted 1/8th notes don't fill the bar when adding tremolo. What does the 12 mean? Is there a way to accomplish the same thing without a 12? Is this a way of writing tremolos that is no longer used?

Thanks, wise wizards. You're the BEST! :D