Your question could be answered two ways. If you want to know when wood stocks were first available for the Colt 1908 Vest Pocket, that was in 1916, when plain and hand checked wood could be special ordered at a premium of .90 or 2.75 respectively.
If you're asking when Colt began offering the machine checked wood stocks with flush medallions for the Colt 1908 Vest Pocket, that was after May 1, 1924 beginning approximately at serial number 340,000. (339,882 has been observed with these stocks.)
Jackson, Yes, the dates for the machine checkered walnut stocks are the same for the 1903/1908 Pocket Hammerless Model. On the custom wood stocks, the date they were first made available is 1912, which is a bit earlier.
The first machine checkered walnut stocks with flush Colt medallions are a bit different than the later variations. They are a bit thinner and Colt's began by using different right and left medallions in these stocks, so that when you look at either side, the Colt horse faces the front of the gun. I suspect that they realized they could save some money by using the same medallion in each stock, so after approximately three months of production, they began using the forward facing medallion in the left stock and the same medallion (which now faces rear) in the right stock.
When the conversion to the machine checkered walnut stocks first occurred, Colt's began using a small end label on the box to designate the type of stocks. Here's an example with the early machine checkered walnut stocks and early box end label: