"It seems odd that someone would order an Ideal without the "oak leaf" engraving".

Although it would be possible to order a graded gun w/o engraving, this is highly unusual; so I agree. Check the serial numbers on barrel flats and water table to make sure they are the same. The gun could be a Field Grade with an added set of Ideal Grade barrels with a vent rib; although if so, the side locks should be stamped Field even though the barrels are roll-stamped Ideal. The only way to know for sure how the gun was factory graded is to request a research letter as Dr. Drew noted. And finally, Field Grade guns ordered with factory fitted auto-ejectors are not rare.