1.) The left stamp in the top row measured with a micrometer is .0038" thick. The right stamp in the bottom row is .0030" thick. The others are all between these two thicknesses, in descending order.
2.) The Scott catalog says that #1 is printed on "thick chalky paper", and #1a is printed on "thin unsurfaced paper".
3.) Where is the breaking point between the thicknesses of #1 and #1a?
(4.) 10-14-13. NEW INFORMATION AND OPINION.
(a.) Thickness is not a good test. The thickest #1 above is .0038". The thinnest #1 is .0033". The thickest #1a is .0036". The thinnest #1a is .0030". The overlap is too big. 16 of the 20 stamps above are in the overlap, and could not be conclusively identified by thickness.
(b.) Shades are not a good test either. Both #1 and #1a are supposed to be exactly the same color, but both have many shades.
(c.) Neither watermark fluid, a good magnifying glass, nor an infra red light will reveal the true difference between #1 and #1a.
(d.) For the best tests read the texts for the scans below.
(1.) The best test is to hold the stamp to the light at a certain angle. The #1 (and #2-3) issued in 1912 on chalky paper has a shiny or glossy finish.
(2.) It also has a "softer" snap.
(3.) The 2 top stamps, #1-2 both have 1912 postmarks. Any stamp with a postmark earlier than 1915 has to be a #1, unless it is a forgery or a mistake.
(4.) Notice that there are many shades of the green, 5-heller stamp. (5.) Exp: do. Ref: R.Schneider.
(1.) #1a does not have a shiny appearance when held up to the light at a certain angle.
(2.) #1a has a crisper, more brittle snap than #1.
(3.) Check the postmark, if the stamp has one. This example does not. Anything before 1915 cannot be a #1a.
(4.) Exp: do, Ref: R. Schneider.