Seal Rings are NOT all rectangular section rubber rings. The most commonly used seal ring is rectangular in cross section and fits to the engine block for one and two piece canisters, and to the bottom of the adapter on the two piece canister (Illustration #15 above). Most filters will include one ring of this size, and sometimes two. Dimensions of this ring are:
    OD=3.16 in   ID=2.90 in   Radial Width=0.130 in   Thickness=0.150 in
You may want to order a spare for this seal, in case you do not get two with the filter (very common problem).
Moss - 435-335   Sealing Ring
V.B. - 4-730   O-Ring

The second ring usually included with the filter is slightly smaller rectangular cross section and has unknown application, not for MGA. You can toss this one in the dust bin. Dimensions of this ring are:
    OD=3.00 in   ID=2.86 in   Radial Width=0.070 in   Thickness=0.120 in
Any part number for that ring is unknown, but you don't need it for MGA anyway.

Sometimes there is a third seal ring in the package that may be the same as the first ring, as you need two of these seals for the two piece canister assembly.

But there is a different seal ring needed that is never included with the replacement filter (not that I have ever seen). This would be item 16 in the first illustration, the seal to fit around the internal filter head in the early Tecalemit filter assembly. Dimensions of the packing gland are:
    0.188 width of the gland (top to bottom)
    3.000" ID of canister
    2.644" Minor Diameter at depth of groove on the valve assembly
    0.178" resulting radial space for the gland.
The gland depth may vary a little depending on tolerances and measurements. This will accept a rubber seal ring either square or round section for 3-inch bore and 3/16" gland. The Ring dimensions need to be:
    Square Section Ring = 3.005 OD,   2.639 ID  , 0.188 section width
    Round Section Ring = 3.011 OD,   2.633 ID  , 0.200 section width
Note that 0.188 round section would not compress enough to make effective seal. The OD and ID may vary slightly (depending on stretch or shrink), but the radial section width must be correct.
    Scarborough Faire has an O-ring seal for this application, AA78 Sealing Ring - Early Valve Assembly. Dimensions are approximately:
    3.05" OD, 2.65" ID, and 0.200 round section width.
The SF ring is slightly tight fit in the groove, and seems a little too tight in fitting back into the canister, but should work (if you can get it assembled).