They changed over time. As originally introduced in the Crass Model the No. 1 had some engraving and Twist barrels. Later in the Crass period Ithaca added a No. 1P which had Twist barrels and a plain receiver with the roll-stamping on the sides. The engraved No. 1 went away in a couple of years and what was introduced as the No. 1P became the No. 1. Early on the No. 2 was the lowest grade to have Damascus barrels, but apparently Ithaca saw a demand for a lower priced gun with Damascus barrels and added the No. 1 1/2 which had the plain receiver with the roll-stamped logo on the sides and came with Damascus barrels. As steel barrels came in Ithaca added the No. 1 Special with Cockerill Steel barrels on the plain roll-stamped receiver. Finally Ithaca added an even lower priced gun the Field Grade with Smokeless Powder Steel barrels on the plain roll-stamped receiver with a half-pistol grip stock. By the time the Flues Model arrived the grades were like this.
Field Grade --
No. 1 Special --
No. 1 --
and the No. 1 1/2 --
which had at some point along the way gotten the zig-zag border engraving around the receiver.
Then in mid-1915 things began to change when Ithaca redid their engraving patterns and the No. 1 1/2 got the bold floral engraving --
and the option of Krupp barrels.
As Europe was plunged into The Great War, the supply of Twist and Damascus barrel tubes dried up and these finely differentiated grades all ended up with steel barrels, and the No. 1 got the zig-zag border engraving that had been on the earlier No. 1 1/2 --
Finally by 1919 the No. 1 Special was dropped and the No. 1 and No. 1 1/2 were combined as the new No. 1 --
and the Field Grade got a capped pistol grip.