Some clues to dating Festus, Acurule and Acumath slide rules.

The Festus Mfg. Co. produced slide rules branded as Acu-rule.

The Acu-rule Mfg. Co. produced slide rules branded as Acu-rule and Acumath.

 Most Festus produced rules are marked as made in Festus, Missouri

 Festus Mfg. Co. is a company name.
Acu-rule is both a company name and a brand name.

Acumath is only a brand name.                        

Any slide rule marked as made in Festus, Mo. was made between early 1938 and August, 1939.
  The first reference to the company was found in an advertisement in the May 12, 1939 issue of the   Jefferson County Press.  I have never seen a Festus made rule with a model number printed on it. 
A rule that matches the description on the 1939 Magnus Manheim price sheet, a 10 J, 10 D, No. 50, No. 100, was one of the first made by the Acu-Rule Mfg. Co. after August 1939.  
(see chapter 1 price sheet link)
  On June 20, 1939 a request was filed to create the Acu-Rule Mfg. Co. as a successor to the Festus Mfg. Co,  A Missouri  charter was granted  April 20, 1940.   The company, however, jumped the gun and started selling as the Acu-rule Mfg. August, 1939.
You may expect to find many variations in the early Festus and Acu-rule produced slide rules. 
The very first used scales printed on paper glued to the wood and finished with a clear coating.  Within the same year Laboatory Specialties was selling the same rules and using the same model numbers but they were now 'white face' rules.  White paint was used on  just the face.  Later the face and top edge then the entire rule was white. See Gallery Retail Price List of 1939. Printing was usually blue or black. Some rules are branded in red and some non-branded.   This was a period of experimentation and during the war (1941 to 1945) a struggle to obtain raw materials. The paper equivelants table on the reverse was one of  the last to go. 

Acu-rule wood rules were made during WWII with some variation in cursors and patterns but basically remained unchanged.
In 1946 a new product line of metal-core rules was announced.  They were branded as Acu-Math  
These magnesium rules were originally painted with nitrocellulose lacquer.  They are distinctive in that a round finger indent was drilled   into each end of the slid. This was later dropped.  By 1948 the metal core rules were vinyl laminated rather than painted.

In 1946 the brand name Acumath and the term Acu Design were first used.
 Acu Design was a design service and not a manufacturing company. 
The name can be found on Acu-rule and other brands.

With the production of the metal-core rules model numbers were printed on the rule.
Some of these models were A60, A80, 90A, A130.  These models are not found in a 1960 catalog and were discontinued before then.  The later magnesium rules have model numbers ending in 11. Example 1311.

A model 12 P was made from an unidentified plastic that looked like a clone of the early metal-core rules
    This rule can be dated the same as the metal-core rules in 1946. It was short lived.

The best estimate for the production of vinyl slide rules made from the laminated Bakelite material is 1948.

Models in production in 1960 were 1500, 1300, 1311, 1200, 1211, 1240, 900, 600, 511, 500, 400, 400B, 550

The model 11 is the only wood rule branded as Acumath.  It should be dated 1946 because of the brand name.

The last catalog available is about 1962.

Models not in this catalog and dropped by then are models : 11, 12, 12P, A60, 90A, A130, 550,   800, A80.     The model 150 was known to be in production before 1964 and the model 150A was produced after 1964.  A late vinyl slide rule was the unique model 4.  It was not made until 1965 and although it is a 10 inch rule it is only as  wide as a pocket rule.  It had a very good magnifying cursor and was the only model sold in a plastic bag. Few were sold and it is hard to find.

The oval shaped Acumath logo was designed by Mr. Muschong after he joined the company in 1947. 
The first record of his joining the company is in 1947 in a StL. City Directory.
It would follow that all rules made with that logo were made late in the 1940s and
would include the entire production of the
Mt. Olive, Illinois plant.
Yellow vinyl film used to produce sight saver rules was first available after 1963
The sight saver rules carry the "T"  designation in their model number. Example 1300T