1. BU14 is only fitted on the front shock absorber on the flanged link bolt that fits in the axle. fitted in the order : -Locknut - locknut - washer BU14 - fibre washer - link - fibre washer - flanged link bolt - axle. BU14 later became 1A5145 (2 per car) and these are still available. (The Austin 7 workshop is one supplier according to their spares list).
I believe the washer is there as a normal washer would be behind a nut, but in this case it is prevented fromrotating so that it provides a fixed face for the fibre washer to but against in the same way that the arm does at the other end of the link. (?).
You say that the link bolt cannot turn as it is keyed to the shock absorber plate.....think you are mixing link bolt lower (flanged (fixed to axle )) and link bolt upper (Pegged) that goes through the arms. (no BU14 on that, just 2 lock nuts and a spring washer and I would put a plain washer between spring and side arm.
HOWEVER note that the spares list does have the rear link bolts, with peg 1A5278 as being listed with a key washer 1A5145 (BU14) !! and spring washers 2K1212 ....so same mechanism but slight difference? !A5278 same for both front and rear dampers.
2. I cant see any reference to BU15 in John Platts spares catalogue or any of the original list of spare parts after Oct 1936 (Pub 1406C) but Pub 1218A Mar 1935 shows BU15 in the picture but does not mention them under the description/number per car/reference number;
however in the 1936 description associated with "side link bolts lower" BU36/1A5142 (the ones that fix to the axle) are:-
key washer(BU14) 1A5145 (X 2)
nuts, front (BU43) 2k3976 (X 2)
spring washers (BU44)2K1211 (X 2)
nuts, rear (BU45) 2k3986 (X 4)
spring washers (BU46) 2K1212 (standard looking split spring washer) (X4)

Spring washer 2K1211 seems to be the replacement part and number for BU44
and 2K1212 for BU46, BU48, BU26 etc. I have no idea what difference between them is other than maybe bolt diameter.

It would seem that BU15 Could be an earlier version of a spring washer.
It would appear that some items may have more than one reference or part number depending on where they are fitted?

faint pencil marks in the original spares lists I have suggests the store-man at the time had some confusion too!

Location: NW Devon

To answer question 1 I believe this is tied up in the comment I made in "Shock absorber link rubber bushes questions". I believe that the rubber bushes must be compressed and that there should be resistance to rotation -- a normal washer would allow more rotation.

Location: Vancouver Canada

the cups at each end of the spring.

i make and supply at 7 county austins.

tony

Location: huncote on the pig