New 6" 24 GA Black Oval To Round Adapter by Imperial Mfg.
model BM0037
item 287755
6" 24 GA Black Oval To Round Stove Pipe Adapter.
Oval end of adaptor measures approximately 7.5" long x 4.25" wide
Adapter measures 7" long
Oval end can be "squashed" down more to make it a little wider than 7.5", which of course makes the width a bit "skinnier" than 4.25".
Part is riveted together with 4 rivets, so rivets could be drilled out to make this part smaller than the above dimensions, by shortening and re-riveting (for custom applications)