Yes they are interchangeable. The 580 is a single shot bolt action and the 581 is a clip fed bolt action.
Push the safety lever fully forward to dis-engage the bolt stop. Then the bolt can be removed rearward.
First take off the stock. Second pull down on the bolt stop pin located in front of the trigger. Third while holding down on the bolt oin remove the bolt.
According to Remington 60,000 from 1967-1987.
No
$ 375
They were made from 1967-1987 and are date code stamped. See related links for details how to look up the manufactured date.
580 + 580 = 1160
35% of 580= 35% * 580= 0.35 * 580= 203
12% of 580 = 12% * 580 = 0.12 * 580 = 69.6
30% of 580= 30% * 580= 0.3 * 580= 174
580
4.7% of 580= 4.7% * 580= 0.047 * 580= 27.26