Original Star powder slides came in four thicknesses. The 0 powder slide (marked on the edge for the bar) and typically found on "Progressive" models is 1/4" thick. The 00 powder slide, the most common, and standard on "Universal" models is 5/16" thick. Star also made powder sides that are 1/2" and 5/8" thick that are typically used for magnum charges. These powder slides are not marked on the edge.
All of the above powder slides are marked on the top with the powder, and the charge in grains.
Star made corresponding powder slide housings marked No. 0, No. 00, 1/2 and 5/8. Star also made spacers to convert a No. 00 housing to handle a 1/2" or 5/8" powder slide.
The 299 designation is not a marking that I'm familiar with, but there are powder slides out there with markings for a number of now obsolete powders.
Bruce Williams