trex1310: The Model 64 was introduced in 1970 with a 4 inch tapered barrel, fixed sights, in .38 special on the medium sized K frame. It is the stainless steel version of the Model 10 Military & Police that has been in production in one form or another since 1899. in 1972 the 64-1 was introduced with a four inch heavy barrel and by 1973 the four inch tapered barrel 64 no dash was discontinued.
The model 64 continues in production today after several engineering changes. It has been the issue firearm for the Detroit Police, Delaware State Police, DC Metro Police, Phoenix Arizona police, NYPD, and Brinks security. The Oklahoma Highway Patrol ordered 750 in 1973 chambered in .357 magnum, this later became the model 65.
Your gun falls in the serial number range for 1972-1973, being D420001 to D510000.
TenmaNeko: The Model 2 Old Army is a single action .32 caliber rimfire revolver with a six shot cylinder. These were manufactured in the serial number range 1 to 77155 between 1861 & 1874. Yours was made in or prior to 1864 as the serial numbers ran to 29359 in that year.
Depending on condition these can be quite valuable. There were also several copies made. I suggest you take some pictures and post them in a new thread, quite a few of us will be interested to see them and you may learn something new.