I have read that industrial microwave ovens operate at 900 MHz rather than the more common 2.4 GHz ovens found in most homes. Why is this? Some examples of this claim: Continuous Microwave Processing for Heating Materials  Comparing Microwave to Conventional Heating & Drying The latter link claims "[The 900 MHz] range allows more efficient penetration of the microwave through the material." Though I don't know why that would be. 
For freezing ice cream or other confections in a refrigerator’s freezer area, a refrigerator tray was a rectangular, shallow, open container. The ice cube tray (often provided with the refrigerator) began, around the mid-to-late thirties, to be fitted with a removable divider so that it doubled as the refrigerator tray. Ice cube trays with removable dividers are referred to as “refrigerator trays” and were designed specifically for freezing desserts. The terms “refrigerator tray”, “freezer tray”, and “ice cube tray” were used somewhat interchangeably for making desserts with these trays, at least during the period after ice cube trays with removable dividers were included with refrigerators and until the included ice cube trays became one-piece plastic trays. The Carnation Fun to Cook Book, for example, has a recipe for “tropical freeze” on page 40; for the final freeze, the recipe is poured into “two small (or 1 large) refrigerator trays”. The drawings accompanying the recipe show shallow, rectangular trays.  Experiences with Foods by L. Belle Pollard includes notes on “making ice cream in the home refrigerator” on page 258.  The ingredients have been combined according to the recipe and poured into the refrigerator trays or molds.  The accompanying photo shows two of these refrigerator trays. They are metal, rectangular, and shallow.  You can also see refrigerator trays used for desserts in this 1951 commercial for a Westinghouse refrigerator. By at least the late 1960s, authors had begun to use the term “ice cube tray” in recipes for ice creams made in the refrigerator. In Vincent and Mary Price’s 1965 A Treasury of Great Recipes, there is a recipe for Tre Scalini’s Hazelnut Ice Cream on page 93 that says to “Pour into 2 ice-cube trays and freeze. This makes a soft ice cream.” Borden’s The Dessert Lovers’ Hand Book from 1969 also contains recipes for making ice creams in the home refrigerator. The accompanying illustration shows the same shallow, rectangular container, but in this case calls it an “ice cube tray”—despite there being no cube shapes in that container as drawn. The ice cube grid has been removed.  Borden’s 1952 Borden’s Eagle Brand 70 Magic Recipes uses the term “freezer tray” for the same Vanilla Ice Cream recipe that their 1969 Dessert Lovers’ Hand Book contains. The recipe is the same; only the terminology has changed. One patent for a refrigerator tray with removable grids for ice cubes is Richard E. Kaufmann’s patent 2,112,027 filed October 3, 1936, found in the Official Gazette of the United States Patent Office Volume 488. The patent is labeled “Refrigerator Tray” and shows a “removable grid for ice cube trays”.  A slightly earlier patent, Louis Brownstein’s patent 2,037,364 from March 17, 1934, is labeled “Ice Cube Tray”, but specifically mentions its use for desserts:  My invention further contemplates the provision of a freezing tray from which the grid partitions and other parts may be readily removed for purposes of cleaning or when the tray is to be used without its partitions, as when it is desired to freeze desserts, etc. in the tray.  Brownstein writes specifically that the grid-less tray may “be used for freezing liquid desserts in one mass”. You can see ice cube trays with removable dividers in this 1954 commercial for a Crosley Super Shelvador. Here’s an example of a later iteration of an ice cube tray with a removable divider, a double-sized Quickube from a Frigidaire refrigerator:  Notes  While traveling recently I acquired Borden’s Dessert Lovers’ Hand Book and Price’s A Treasury of Great Recipes. The reference to ice cube trays in these two books provided the necessary clue to answering my own question—that a refrigerator tray and an ice cube tray were at least in some instances the same thing. This led to finding Kaufmann’s patent on a refrigerator tray that was clearly an ice cube tray. Borden’s 1969 Dessert Lovers’ Hand Book also includes instructions for making ice cream in specialized ice cream churns; the 1952 70 Magic Recipes only has recipes for making ice cream in the refrigerator. The archived version of Borden’s 70 Magic Recipes that I linked to is supposedly dated 1956; I’m not sure how archive.org arrived at that date; the only obvious differences between the archived version and my copy appears to be that (a) mine has a copyright year of 1952 on the inside front cover where the archived version does not include the year in the inside-front-cover copyright statement; and (b) the inside back page on mine has what I think is a print date of 8-52 and a version of E-56 where the archived version has what I think is a print date of 5-63 and a version of E-56 Rev. The Quickube ice cube tray has levers for more easily removing the grids from the tray, a feature mentioned in later patents such as F.W. Sampson’s 1946 patent 2,573,066 and E.H. Roberts’s 1956 patent 2,763,997 both of which are fascinating historically but don’t address this question.  