Yeah the Dodge's came in 241, 270, 315, and 325. The poly's were 270 up to 325.
But, if the serial starts "D5xx" and the next digits are anything other than "00" or "01", as in D500 or D501, its pretty much a 270. The fact that it's a poly with those numbers pretty much makes it a '55 270, unless someone has swapped poly heads onto an earlier '53 or '54 270 hemi block (unlikely). And even in that case it's a poly now, the blocks were the same anyway.
They made the poly's all the way up to '58 and in displacements up to 325, but none of those serials would start with "D5". Or, well, they shouldn't based on everything I know. The serials on the early hemi's were even less regimented/documented than the numbers and options on our musclecars and there are a LOT of different serials, since the serials were different for the same engine if it was in a different vehicle, so I try not to say that they didn't do something. Just that I haven't seen/heard of it. A casting number from the block would narrow it down a more.
But, my money would be on it being a '55 270.