There's really no telling the panel rating by looking at it. I've seen 100, 125, 150, and 200A panels that look like that. I've also seen a 300A panel that was similarly configured (breakers on top of each other instead of side by side).
The one thing we can say with relative certainty is that you can backfeed that panel more than 40; it would be a supply side connection. You can backfeed as much as the service is rated for, which in this case could probably be judged by the service conductors as Dennis suggested. I've also had a couple cases involving missing labels in tract developments where the AHJ had us look around at neighboring houses that clearly had all been built with the same equipment.
Of course, as has also been mentioned, you'll need to do some other work to have a place to land those breakers, or get lucky with whatever panelboards those breakers are feeding.