That is covered in the rules listed on this site.

Section 2.91:  Sandbags and Rests 
A sandbag or shaped rear rest sand filled bag made of leather or canvas may be used under the toe of the butt stock. An anti-cant device (bob-sled) similar to the Pope style muzzle rest or the one described in Mann’s book is permitted. Also a spirit level or sight attached to the barrel is permitted. A simple stop may be attached to the front rest as an aid in realigning the rifle for the next shot. This may be contacted by the muzzle rest or the forearm at the discretion of the shooter if used at all. The front rest assembly and rear bag may not be connected mechanically or tied together. The shooting bench top surface is the only common connection between the two that will be allowed. 
   

I presume you are talking about this type of rest.
Kurt

  If you can make one that violates the ASSRA rules, I'd sure like to see it!

Dales wrote on Mar 22^nd, 2015 at 6:44pm:

  That is correct.

The fly in the ointment in the  rule on anti cant device I think would be:

An anti-cant device (bob-sled) similar to the Pope style muzzle rest or the one described in Mann’s book is permitted.


I am thinking that Popes sled and the one described is only around 3 or 4 inches in width.
The question on the width of the forearm made me think a bit on the subject.

Then there is the catch phrase "in the spirit of" which is real hard to define.
In the context of this specific thread    I'd say that the first example is less likely to be so than the second. 

Remember too that the "traditional class"as such is more of a badge of honor in ASSRA (as practiced at Etna Green at least).  Other than the "!/4 Bore" matches which are "side matches",  I don't believe that we currently have any matches that are specifically restricted to "traditional-class rifles".  We simply self-designate if we are shooting a "traditional-class rifle". 
I suppose someone might challenge another shooters self-identification of a specific rifle or set-up in which case Schuetzenmeister Jeff would have to make a common-sense pass/fail ruling on it.  I'm not aware of that step being required yet.  
  
Other ranges and single-shot shooting orgs. may have other rules and practices.  If taking a specific setup that might be questionable to a match;  I'd certainly check with the match organizers about their "traditional-class" interpretation and practice.

The Pope machine rest replica we have at EG has a "muzzle rest" that is little bigger than an old lightning rod cable clamp.  I've handled it a number of times and posted photos of it in earlier discussions.  But I have not actually measured the width of the muzzle clamp base certainly its not much more than 3 inches.
I like the muzzle clamp-on rests I believe they can be a real help when shooting off-hand style stocks "from the bags"   especially for those of us shooting the Euro-schuetzen rifles off the bench.
       There is another factor to evaluate.  Too long or large a contact surface between the muzzle clamp ("runners" if you will)and the flat rest surface may become counter productive as the front rest is height adjusted for elevation.  the smaller rounded contact points probably function better with less binding.

Many picture of the old timers rests had a "V" notch in the center of the front.   I found using the notch against a peg with a heavy shoulder into the rifle was the most accurate.  YMMV