I realize this thread is about 1-1/2 years old, but I have a couple of answers to the OP's questions .
I have a roll of Kodachrome 25 in 828 size. It's expiration is dated Mar 1977, the backing paper is clearly marked '828 Kodachrome 25'. I have looked at rolls of KM 828 offered on Ebay and have not seen any with a later date. More interestingly, the roll of film has the infamous 'white band of death' surrounding the unopened roll. (For those of you that don't know, Kodak replaced the regular Kodak yellow colored band securing a fresh roll of paper backed roll film with a white printed band stating the discontinuance of that type of roll film in that size when the film size/type was discontinued by Kodak). The KM 828 roll of film states on it's white band: 'Eastman Kodak will not manufacture Kodachrome/Ektachrome film in this size after 1975'.
It's very interesting that my roll of KM 828 still comes with the film in an aluminum can. My October 1973 Modern Photography has a short article announcing that Kodak was replacing the old aluminum film cans with the new plastic cans (which at the time had black caps and gray colored bodies-this was changed roughly about a year later to the gray caps and black cans which Kodak still uses).
It's also very interesting that Kodak offered Kodacolor in 828 size 8 years before 135, Kodacolor 35mm was not offered until late 1958 (it's reviewed in my Oct. 1958 Modern Photography).
My 1973 printing of the Kodak book 'Adventures In Existing Light Photography' lists 828 film as being available in Ektachrome-X, Kodacolor-X and Verichrome Pan (The ASA 25 versions of Kodachrome are not listedanywhere in this book,; Kodak must have thought ASA 25 too slow to consider for listing in this book). My 1976 revised printing of the same book lists Kodacolor II as being the only film in 828. VP and EX are gone, CX was replaced by C II.
I also have a listing of old Kodak roll film sizes that was posted on the Internet on an old AOL site (year 2000?) which is now long gone. It lists the year of introduction and
month/year of discontinuance of old Kodak roll films sizes 101-119, 121-130, 616, 620, 828 and 35 (not 35mm!) 828 film is listed as discontinued in Mar 1985 (which was Kodacolor II, the last film made in 828).
It was August 1974 that Modern Photography had a front page article on the then
new K-14 Kodachrome. I would think that KM in 828 only became available at the end of '74 as existing stocks of the old KII 828 size were depleted. With KM 828 being discontinued by the end of 1975, I suspectK25 828 was really only available for about a year. Amazing that Kodak went to all the trouble to manufacture this film size for only about a year-talk about loyalty to their customers!
Most people don't know that when Kodak replaced an old film with a newer type they
didn't replace all film sizes all at once. The most popular and fastest moving sizes got
replaced first, then less popular sizes, with usually the old roll film sizes becoming
available st. For example, a casual search on the Internet brings up the fact that
Kodak replaced the C-22 process Kodacolor-X film with the new C-41 process
Kodacolor II film in 1972. This is only partially true. Kodacolor II was brought out in
summer 1972 with the roll out of the brand new 110 size cameras. Kodacolor-X film was not replaced in 35mm until early 1974. Modern Photography tested
35mm. Kodacolor-X vs. Kodacolor II in their April 1974 issue/
I think it took a long time for CX in 135 (and certainly roll film) to get out of the pipeline-probably sometime in 1975.
Last year, I went to a website called newspapers(dot)com and through a paid
subscription was able to download many camera department film ads from K-Mart, all
dating from the 1970's (which was the era i was interested in).
I have some K-Mart camera dept. ads from March 27 1975 where they are selling
Kodacolor-X in 35mm. size, and in the same ad Kodachrome 25 in 35mm AND the new Kodacolor II in 126 size! (You may not believe it but 126 size color print film, was more popular in the late 60's and early 70's than 35mm. color print film.) To me, this
ad supports the idea that C II showed up on shelves in 126 size before it showed up in 135 size.
I think it's a safe bet to say that in 1975, Kodak's least popular roll film sizes still in
production were 116, 616 and 828 and I think the new Kodacolor II did not appear in
those sizes until 1975-probably late '75!
I did see on Ebay a few months ago, someone offering a roll of Kodacolor-X in 828 size, it had an expiration date from 1976 (can't remember the month but it was likely Spring 1976). (Sorry to get off on this bunny trail).