Hi Rob

The reason your Cessna 150 doesn't have or require logs like the homebuilts is that they (The type certificate holder, Cessna) has a production system and QC system that satisfied the FAA that Cessna could build aircraft identical (or nearly so within limits defined by Cessna and agreed to by the FAA) to the first one certificated of that type.  All production stages have to be performed by a Cessna emplyee and signed off by an inspector and any stage may be inspected by the FAA or a representative.  Your plane will have a log on record at Cessna that showed when each part was built and signed off, what engine it had in it, weight and balance, etc.

Dan Newland