I expect the washer under the head of the front guard screw would be described as a locking washer, at least here in N.A., as the function of the two raised edges is to inhibit rotation, not impart spring tension.
The washers used on the No32 bracket clamping screws are a true spring washer in that they have two full "rotations" and do indeed function as a spring, though the utility in that context is debatable since whatever tension they impart would not be enough to hold the bracket securely, and when the screws are done up completely the washers are fully compressed to the point that they function as plain washers.
The collar you refer to AFJon has as I understand it, two functions: to maintain the correct functioning of the trigger and to prevent the crushing of the bearing surfaces under the barrel reinforce and the adjoining bottom surface of the receiver, and the opposing surfaces under the front of the trigger guard.
The wood between those two opposing steel surfaces needs to be under a degree of compression to avoid the looseness you describe, but if the collar is too short and the wood is over-compressed it will obviously crush over time and looseness result.
If the bearing surfaces in the forend seem relatively unmolested and show signs of even contact, you probably just need to shorten the collar slightly. BUT if you have reason to think the rifle has not been molested/altered over the years, the best first step would be generous applications of
linseed oil![icon](autolinker/images/link6.gif)
over a week or so to swell the wood back closer to its original dimensions.
We tend to forget that the vast majority of our generations-old rifles are dry as dust and need this treatment regularly to avoid significant shrinkage.