In order to save space (yeah, I know it's long anyway), I have copied the most pertinent sections of the applicable codes. You may wish to look at the full language of the codes which are available online. (All emphasis below is mine.)
You were charged with:
811.265 Failure to obey traffic control device; penalty. (1) A person commits the offense of driver failure to obey traffic control device if the person drives a vehicle and the person does any of the following:
(a) Fails to obey the directions of any traffic control device.
(b) Fails to obey any specific traffic control device described in ORS 811.260 in the manner required by that section.
ORS 811.260 says: 811.260 Appropriate driver responses to traffic control devices. This section establishes appropriate driver responses to specific traffic control devices for purposes of ORS 811.265. Authority to place traffic control devices is established under ORS 810.210. Except when acting under the direction of a police officer that contradicts this section, a driver is in violation of ORS 811.265 if the driver makes a response to traffic control devices that is not permitted under the following:
(11) Stop signs. A driver approaching a stop sign shall stop at a clearly marked stop line, but if none, before entering the crosswalk on the near side of the intersection or, if none, then at the point nearest the intersecting roadway where the driver has a view of approaching traffic on the intersecting roadway before entering it. ORS 810.210 says: 810.210 Placement and control of traffic control devices. (1) The Oregon Transportation Commission is vested with exclusive jurisdiction over the installation at railroad-highway grade crossings of signs, signals, gates, protective devices or any other device to warn or protect the public at a railroad-highway crossing. The commission is granted exclusive authority under this subsection to determine the character or type of device to be used.
(2) Each road authority shall place, maintain and control traffic control devices used upon its own highway as the road authority considers necessary for the safe and expeditious control of traffic, necessary to carry out the provisions of the vehicle code or local traffic ordinances or necessary to regulate, warn or guide traffic. The commission shall act as road authority under this section in lieu of the Department of Transportation. The authority granted under this subsection is subject to all of the following:
(a) All traffic control devices erected and used under this subsection shall conform to the state manual and specifications established under ORS 810.200.
(c) Only the commission has authority over a state highway whether or not the state highway is within the jurisdiction of another road authority. No traffic control device shall be erected, maintained or operated upon any state highway under this subsection by any authority other than the commission, except with the written approval of the commission..
ORS 810.200 says:
810.200 Uniform standards for traffic control devices; uniform system of marking and signing highways. (1) The Oregon Transportation Commission may exercise the following authority with respect to the marking, signing and use of traffic control devices in this state:
(a) The commission shall adopt a manual and specifications of uniform standards for traffic control devices consistent with the provisions of the vehicle code for use upon highways in this state.
(2) The authority granted under this section is subject to all of the following:
(c) All traffic control devices placed or operated in this state shall conform to specifications approved by the commission.
Under the Oregon Administrative Rules #734-020-0005, the 2003 Edition of Federal MUTCD and the Oregon Supplement to the Manual on Uniform Traffic Control Devices dated July 2005 have been adopted as the specifications of uniform standards for traffic control devices for use upon highways within the state.
The Oregon Administrative Rules Therefore it would appear that if the sign does not comply with the either the 2003 Edition of the federal Manual on Uniform Traffic Control Devices or The Oregon Supplement, then whichever road authority placed the sign did not have authority to do so and it is therefore unenforceable.
Manual on Uniform Traffic Control Devices
(1) In accordance with ORS 810.200, the 2003 Edition of the Manual on Uniform Traffic Control Devices with Revision no. 1 Incorporated, dated November 2004 (U.S. Department of Transportation, Federal Highway Administration) is hereby adopted by reference as the manual and specifications of uniform standards for traffic control devices for use upon highways within this state.
(2) The Oregon Supplement to the Manual on Uniform Traffic Control Devices dated July 2005 is hereby adopted by reference as a register of deviations to the 2003 Edition of the Manual on Uniform Traffic Control Devices.
Also note, that there is a further requirement that if this is a state highway (you have to check the official designation to be sure) and if anyone other than the Oregon Transportation Commission placed the sign, they would have had to get approval from the OTC (see 810.210 (c) above). If this was not done properly, that would be a further point in your favor that the sign was not legally placed and not enforceable.
Note also that there is an exception in the code for signs placed before June 27, 1975, so in the hugely unlikely case that this sign was placed before that date (and has never been replaced) then the above restrictions might not apply.
Hope this helps.