'57 Chevy pick up

Hubby & I just got a '57 Chevy pickup that's been parked a few years. There's a number on the engine D909D, what does it mean? We Googled it but came up empty handed. Any ideas???

Is this a number in the block casting? If so, where is it located?

Is the motor an InLine 6 cyl or a V-8. Do you think it's original to the truck?

Hmm, the usual lack of necessary info.