For the past several decades, Lyman has been THE major manufacturer of bullet molds for blackpowder shooters. (I spelled it as Lyman Mould because that's the way Lyman spells the word for a metalcasting "mold.") Here's a link to a chart showing some (but not all) of the many-many Lyman cast-bullet varieties. The closest match-up for your bullet on that chart is #445369 and #445599 ...but I don't think they are .54 caliber, based on the weight stated in the chart. Perhaps yours is an obsolete version in .54-caliber, no longer being listed in the sales-chart.
Ol' Buffalo Bullet Mold Tables
If you wish to pursue the subject further, send photos of your bullet to the good folks at
www.castboolits.gunloads.com. You'll find a staggering amount of info on modernday Blackpowder Shooting/Hunting and modernday cast-lead bullet molds there.