Richard,
Thank you very much for taking the time to post your pictures.  Fantastic looking rifles   I noted in the Stevens advertisement you posted the reference to the 417 being called the "Senior Ideal" and the 418 be referred to as the "Junior Ideal".  I think people often mistakenly refer to them both as Ideals without noting the difference.  Thanks again for your input.  Terry

I have never seen anything in the Stevens catalogs that would indicate factory checkering was an option on 418s.  The rifle on GB appears to be checkered nicely, but the pictures are not sharp enough to show good detail.  It has a replacement butt plate and the pistol grip cap has been added, so my bet would be that the checkering was done by a previous owner.  

I don't know how far Stevens went regarding special order features in the 1930s.  Their catalogs indicate little in this regard.  They do say that Stevens was proud to make manufacturing improvements that would keep the cost of their firearms "...at  prices which are appreciated by thrifty sportsmen."  This means to me that their products were fairly standardized to keep the costs of production down.  There were a few options mentioned in the catalogs relative to sight combinations, calibers, loop levers and, in the case of the 417, a barrel that was longer and heavier.

From my own 418 and 418 1/2 rifles, I can see no variation in butt plates, early or late.  They have a flat steel plate that has horizontal serrations in the middle and is smooth on the ends.  One exception is a rifle with a Favorite hard rubber butt plate, a pistol grip cap, an ivory inlay in the forestock and coarse checkering that was obviously not done at the factory.  

I have two early (2-digit s/n) rifles with identical tapered forestocks, something not mentioned in the catalogs, and the rest have straight forestocks.  I like the tapered ones better.

All of my 418s are drilled and tapped for scope blocks.  More than half of them also have dovetail slots for rear barrel sights, something not mentioned in the catalogs.  The slots are located in the same position on the barrel.  Special order?  I don't know.

There is some variation in front sights and sling swivels among my rifles, but these are things anyone could change to suit themselves.

I guess this hasn't answered the question of special order features, but it does put a little more information into the mix.  Maybe someone else has the answer.