I meant to post on this subject awhile ago, but since it has come up, I thought to post my current thoughts. According to the ALCO production information that I have, the only domestic order that was on the books and was cancelled were 15 C430s to be built for PC. They were to carry construction numbers 6019-01 to 15 and road numbers 2060 to 2074. I do not know what the Sales Order number was.
As for the other domestic orders that are sometimes listed as cancelled, aside from the fact that they do not seem to have ever been given construction or production numbers, I suspect that they were proposals by ALCO to sell those units to the railriads, and over time, they morphed from proposals to orders. A look at some of these orders in context, and availiable evidence, helps make the case that they never were.
SP&S orders for C636s and C415s. This is plausable in that SP&S was a strong supporter of ALCO, but consider this from Burlington Northern and its Heritage by Steve Glischinski: "Late in the year, four more C636s arrived, along with two unusual C415 1,500hp center-cab swithchers. They would be the last ALCOs received by SP&S. In 1969, with ALCO soon to close its doors, SP&S ordered six 2,000hp GP38s from EMD." Even though ALCO stopped production in early 1969, the decision to order locomotives takes some time, so the decision to purchase the EMDs was proably made while ALCO was still in the business. It is also intertesting that SP&S did not by GE locomotives, since they would have been much more familiar with GE equipment.
GB&W C430s. When I researched my artile on the C430 in the mid 1990s, the only additional order for C430s that was cancelled that I could confirm was S.O. 21278 for 10 units to the Rock Island, construction numbers 3470-01 to 10. As for the GB&W, the numbers do not make sense, as they had never purchased more than 2 or 3 diesels at a time, and all of the Century Series units they purchased were single unit orders. A check thru Green Bay and Western by Stan Mailer shows no mention of additional C430 orders, but does have this regarding the roads lone C430: " Boxy and ungainly looking, the 315s distinctive profile was a regular on the long, single train daily that constituted GB&Ws 1970's way of life." So why would there be a need for that many C430s?
Additional C415s fro the Rock Island. Other than the lack of Sales Order and Construction numbers, I do not have any direct proof that RI did not have C415s on order. However I have talked to people who have related that RI was interested in additional C415s at one time, but it aparently never went anywhere.
New Haven C415s. With the last of the 10 C425s that the NH ordered arrivning in early 1965, it seems odd that they would go back to ALCO almost 4 years later, knowing the condition ALCO was in. Also, with the NH entering PC on 1-1-69, I find it hard to believe that PC would have approved the purchase of 10 new units at that time.
As for exports, the only cancelation at the time of the shut down I can find listed is 2 assigned construction numbers 6020-01 - ? and 6021 - 01 - ? for West Pakistan. The interesting part of this order is that listed as the llocation of construction is a D, rather than an A for ALCO or an M for MLW.
Hope this helps, and if anyone has any other hard evidence, I would be most interested in seeing it.