The Ike dollar was a major challenge for the mint's production capabilities. The mint had not struck a dollar-sized coin in decades, and the copper-nickel used for the circulation strikes was much tougher than the 90% silver previously used in dollars. There probably hasn't been a more difficult coin for the mint to produce before or since.
When the coin was first being readied for production, it was found that the original reverse design was of too high relief to be able to strike up fully, given the limitations of technology and budget at the time.
To address this problem, a low-relief (Type 1) design was created in addition to the high-relief (Type 2) design. For the first year of issue in 1971, the low-relief Type 1 was used for all business strikes (Philly, Denver, and the silver uncirculated San Francisco Ikes). The high-relief Type 2 design was used only for the more carefully produced silver proofs.
In 1972, the the Type 2 high-relief design made it to the 1972-S uncirculated in addition to the 1972-S proofs. The 1972-D coins retained the Type 1 low relief reverse throughout the year.
The 1972 Philadelphia production is more interesting. For most of the year and many millions of coins, the Philly mint produced coins struck with the Type 1 low relief.
But sometime in 1972, a single Type 2 high-relief die was used to strike some 1972 Philly coins, either by accident or as an experiment to see how it would hold up to the stress. It is widely presumed that this was a proof die pressed into service for business strikes. Based on die life it is estimated that less than 100,000 of these 1972 Type 2 coins were struck.
Late in 1972, better die steel became available, and taking advantage of this a new modified high-relief relief design (Type 3) was created. This third (and final) type was used for the remainder of 1972 Philly production (many millions of coins) and for all coins of subsequent years, business strikes and proofs alike (with the exception of the bicentennial issues).
It is therefore only the 1972 Philadelphia coins that are found with more than one reverse type. To summarize:
1972 Type 1 is very common in typical condition, but is the most poorly produced date of the entire Ike series. Typical condition coins are simply awful, making gem condition coins very difficult. Despite the high mintage, time may well prove this coin to be the most difficult of all coins to find in the highest (MS66+) grades.
1972 Type 2 is the king of Ikes, it's the series' "3-legged Buffalo" or "55 doubled die Lincoln", created with only one die. It enjoys a combination of scarcity and popularity and is one of the few business strike Ikes to bring a premium in any condition. The Type 2 is generally found nicer than other types (if you can find one at all), but with the low mintage there is just a tiny fraction available to search. In lower grades there is no question the Type 2 is the most valuable type, but in the highest grades the Type 1 is a worthy contender.
1972 Type 3 is by far the easiest of the three to find in gem grade. However, "easy" is a relative term when you are talking about early Philly Ikes.
Keep in mind that the 1972 Philly date, even with all three types combined, has for years proven to be the most difficult of all Ike dates to find in gem condition. In fact only a handful of PCGS MS66 examples have ever been graded, and of course none higher.
Attribution of the three types can be done with the naked eye or low magnification. There are numerous differences, but the Earth on the reverse provides the easiest positive identification.
First look at the outlines around the continents of the earth and go from there, as follows:
Real-life examples of each reverse type are probably already in your collection. The best examples to look at are the first three years of the readily-available silver uncirculated "blue pack" Ikes. The details are generally well-struck, and the dates conveniently correspond to the types as follows:
The Ike series is still not widely appreciated, particularly among dealers. Cherry-picking opportunities abound for the sharp-eyed enthusiast, and possibly finding a Type 2 for just a few bucks is one such opportunity.