I just weighed a 61 3/4" length of the stuff TIDI sent me, and it weighed .9 ounces on my digital scale, and 23 grams on my Ohaus triple beam scale.
I thought I had several boxes of old stuff, but when I made my last attempt at organizing things, I must have ditched the boxes so more would fit in the rubber maid tub! But I did have a box with a few 24" by 36" stuff, it was labeled GM and "Packaged by Fox Manufacturing. One of those sheets weighed 9 grams. I pulled a sheet of paper from a SIG Super Chipmunk kit that measured 24" by 36" and it weighed 12 grams. I would have to do some SERIOUS digging to see if I have anything that I would guess to be heavy or SGM. I have been collecting this stuff for almost 40 years and quite a large stash of tissue, silkspan, and silk, and one of these days I'll spend a day and sort it out, but right now it's just all heaped together in a couple of rubber maid tubs.
My conclusion is that silkspan could vary from batch to batch if weighed accurately. Once I get the sample from Terry to examine, I'll have a good idea of what he has. I suspect it's a different part number that on the TIDI web site is called "every day". The stuff that is the 916213 part number is called "choice." My feeling is that the stuff I got got from TIDI is maybe just a touch heavier than your average GM grade silkspan. If you wanted a more heavier covering, just double cover with it. But as it is right now, I think it's good stuff equal to K&S GM or SIG medium silkspan.
Type at you later,
Dan McEntee