Provided that the Dolnet is Kosher intonation-wise , the altos are very good indeed.
A Bel Air alto is beautiful horn to play and can have a nippy action. The Tenors are very good too but they are on the heavy side and their keywork is noticeably less ergonomic than a modern horn, but that is a price that most Vintage horns player are willing to pay.
Of course the ones that one should be on the look out for are the M70's, the most modern and special horn that Dolnet ever built.
700$ is not a lot of money for a good saxophone!
There is no reliable serial chart and, my guess is that many horns were produced and were left unsold throughout the period of the WWII (probably hidden from the Germans who would have used the metal) and found their way to the market after the war making a very difficult to ID the horns exactly (and would explain the HP like the one I bought

which were definitely in use in Marching Bands al late as the late '70 in Holland and Belgium)