The list number should be on the front of the choke tower. It looks like an OEM older Mustang carburetor. If so, it's a 450 vacuum secondary. Would be perfect for a slantif that's what it is. Two giveaways. One is the Ford kickdown linkage as already mentioned. The other is the anti run on valve bracket just ahead of the linkage. The 450 Mustang carburetor had both.