This was just shared with me by a gentleman on The High Road forum.

"According to Frank deHaas, author of "Bolt Action Rifles", Browning used the Sako L-579 action for .243, .308, and other similar length cartridges. The L-461 action was used for .222 type cartridges. The FN built No. 5 Magnum action was for .30-06 length, or longer, rounds. Basically it involved opening up the receiver and lengthening the magazine box to accept the longer H&H cartridges, along with changes to the bolt head and extractor to deal with the belted magnum case.

FN receivers were serialed numbered on the right side of the receiver ring and under the bolt handle stem. They would also have the Belgium proof stamp on the receiver ring, MADE IN BELGIUM stamped on the lower section of the receiver (where you might also find some FN inspectors marks), and possibly a small FN within a circle on the right side of the receiver ring."

It doesn't answer all my questions but it helps. Once the rifle is in my hands I'll be checking on the items he describes.
Keith