The ball on the left is regular DMC size 8 perle cotton. The red spool on the left is Corticelli's silk purse twist, and on the right is Collingbourne's "silk finish" purse twist (some sort of mercerized cotton). Although the two spools are roughly the same size, Corticelli's has 150 yards while Collingbourne's has 100 yards.
So what is purse twist? Five years ago I spent a lot of time trying to figure that out. The vintage purse patterns I was reading made it obvious that it was something close to perle cotton or silk beading thread, but trying to determine the exact equivalent to what is available today was confusing. The bead knitting patterns either don't state a specific size, or simply refer to "coarse", "fine" or "very fine". Same with the beads by the way; few references to exact sizes.
I did finally manage to get my hands on a couple spools of original purse twist and they're pictured up above. The red one is a serious case of over spending on ebay, but the green one kind of makes up for it since it was in with a bag of old thread spools in a junk shop. I figure it balances out to about $20.00 apiece. They're not easy to find and if you have any you're willing to part with I'm definitely interested!
By comparing the vintage threads with the modern DMC perle cotton I managed to figure out a few things. While all three threads pictured above are about the same diameter, both purse twists are spun much tighter. The perle cotton is made up of 2 plies, or strands, and each ply is clearly visible in the completed thread. The purse twists are each made up of three plies and spun tightly enough so that the individual strands are not as visible in the completed thread. The triple strands and tighter twist would make a stronger thread than the perle cotton, which is something to bear in mind. It's possible to re-spin the perle cotton and get a thread closer to true purse twist. I have a spinning wheel so it's a pretty easy project, but not one I'd have the patience for without the wheel:)
As far as sizing goes, even today there is little consistency between manufacturers. At least we have a set method of assigning sizes and don't have to depend on "fine" or "coarse". From studying the antique beaded purses I determined that most of the bead crochet and bead knitting was done with thread about equal to size 8 DMC perle cotton. Although many of the vintage beaded purse patterns specify silk thread most of the ones I have seen are done with cotton thread. The beads used in the 1920s are much larger than the ones used in earlier bead knitted bags; about the size of a modern size 10 seed bead.
Basically, the finer the thread and the smaller the hook or needles, the less the thread will show on the completed bag. Sometimes I have to play around with beads, hooks or needles, and threads to get the right proportions. In the case of bead patterns which depend on the un- beaded background stitches (as in bead crochet purses) to form the pattern it can be a little frustrating to coordinate the thread, bead and hook sizes so that the beads lie close together but the unbeaded spaces are also well defined.