Yes, the Ultra-light has an aluminum receiver, the serial nbr will start with ult- or utl- , the regular Featherlight has a steel receiver of course. The Ultra's command a bit higher price tag as they are not as common.
What model # was the auto loader 20 gauge Ithaca and is it a good handling shotgun?
Ithaca itself put out a Model 51 autoloader. From what I read, it has some finicky issues that you have to stay on top of maintenance. There are a couple parts that once they get a little worn will cause breakage if they are not replaced on a maintenance schedule.
I am not familiar with all the firearms that were sourced and marketed by Ithaca (such as the SKB's) so one of the SKB's might have been offered in a 20.
I would suggest joining the Ithaca forum and doing some reading and asking some questions, there are folks there that collect and know about all the different models. Me I am into the Model 37 pump guns.