That Keats does describe a composite urn is certainly plausible as free-style urns of the type Keats appears to describe usually depict only one scene that runs continuously around the circumference.  In fact, it is interesting that Keats chooses to have these varying scenes as he includes, then, images of Nature and human life.  Particularly significant is the image of the lovers, who can never kiss although they will remain forever young.  For, to kiss, to marry and be fertile is what perpetuates the race. But, Keats's beauties are denied this consummation of their love, signifying the limitations of Art as compared to Life:

Fair youth, beneath the trees, thou canst not leave
Thy song, nor ever can those trees be barre:
Bold Lover, never, never canst thou kiss,
Though winning near the goal....

Likewise the Spring branches of trees can never shed as the "happy melodist" must forever be piping songs anew.

John Keats’ Ode on a Grecian Urn probably describes not a specific urn, but blended scenes from the Parthenon Frieze which was housed in London as part of the Elgin Marbles looted from Greece by Lord Elgin and other works. Elements described in the poem include men chasing nymphs, pipers, a young man beneath a tree, and trees. Another part of the image includes a garlanded cow being led to a sacrifice by a priest, a procession, and a sacrificial altar. This is a picture of the Panathenaic Festival, and the part of the freize representing it is in the British Museum.