In late 1951 Parker Brothers changed Monopoly's slogan to "Parker Brothers Trademark Name for its Real Estate Trading Game" (it's not known if any of these were released in 1951 or if it was early 1952). The rules have this new slogan under the name "MONOPOLY" on page one of the rules. The common editions still include the blue#6, The black #6, the green #8, and the white #9. The rules still sport the now familiar circles, diamonds, and stars, but interestingly the blue #6 is back to seven tokens and rules with squares. There is also a new copyright date - 1936, 1951.
Changes to the rules begin on page three. "As you build evenly you must break down evenly when you sell houses back to the bank." is added to the "HOUSES" section. The "MORTGAGES" section has been rewritten on page four as has the "BANKRUPTCY" and "MISCELLANEOUS" sections. For the first time "CLUE" is added to the list of other Parker Brothers games. The short game rules add the 1951 copyright and drop London from the list of cities. I believe this was a very short run set of rules as it was replaced almost immediately. The money distribution is also changed in these rules: two $500, two $100, two $50, six $20, five $10, five $5, and five $1 (this would remain the distribution until 2008).
The editions available changed in 1952. Both #6s ceased production and were replaced by a red popular edition that had the same box graphics as the #8. The rules were also changed with the edition change. The new rules have copyright 1951, 1952. There seem to two sets of rules from this time: star rules for the #9, and a set with no symbols for 3 to 8 players.
The picture at the top is changed slightly on the rules with no symbols -- the "Trade Mark" under MONOPOLY in the center of the board is removed. These rules have been completely retyped, but the only changes I see are the "SALARY" section (expanded and clarified) and "COMMUNITY CHEST" is in bold letters in the "JAIL"section (a mistake since the Patent Pending rules). The short game rules remain the same except the Parker Brothers logo is removed from page one. In 1953 the patent number was dropped from both sets of rules.
In 1954 the editions were changed again. The small green and red boxes were replaced with slightly larger red and green boxes with different graphics. The rules were changed to one set with copyright dates of 1935, 1936, 1951, and 1954. The picture at the top has "Trade Mark" once again. There is now no mention of the number of players. "(Hat, Shoe, etc.)" is removed from the "PREPARATION" section. The line "(This rule holds true even though another lot of that colour-group be mortgaged.)" in the "ADVANTAGES TO OWNERS" section is changed to "(This rule holds true for mortgaged lots even though another lot of that color-group be mortgaged.)". In "THE BANK" section "also all" is added before the word "taxes". In the "HOUSE" section"still" is added before "Double Rents". The line about breaking down evenly is moved to the end of the section. Under "SELLING PROPERTY" the section about Houses and Hotels has been rewritten. The "BANKRUPTCY" section has also been rewritten. There are a couple changes just in the lines at the bottom of page four, and San Francisco and Chicago are added at the bottom of the page. The sort game rules with San Francisco exist both with and without the Parker Brothers logo on page one.
The only difference in the 1955 rules was the inclusion of "MARCA REGISTRADA" under the word MONOPOLY on page one.
The 1956 rules are identical to the 1955 rules except that Atlanta is added to the cities on the bottom of page four. These rules were used until sometime in 1957.
In late 1957 the rules were changed once again. The last copyright date remained 1954 but the rules are rewritten entirely. Most of the changes are subtle, making them a little less wordy. The font on the title "MONOPOLY" is smaller and bolder, but the biggest change I see is that "FREE PARKING" is finally mentioned. It has its own section that states nothing happens there. A new two-sided short game rule sheet is also introduced. This one finally mentions the time limit game. These rules were in use until 1961.
The next set of rules was copyrighted in 1961 and made it into the last of the yellow box #9s. These rules are different at first glance -- the picture of the game is removed from the top of page one. The only other changes to the rules are the inclusion of the phrase "at any time" on the first line of the "MORTGAGES" section on page four, and the cities listed on the bottom of page 4 are reduced to just Salem, Mass. The short game rules remain the same except for the inclusion of the 1961 cpoyright date. These rules remained in use until 1973.