Thanks, all, for your replies. When I ordered the replacement cylinders I was not just a MF newbie, but a tractor newbie. This is my first tractor. But I was no newbie at locating mechanical parts on the net. It could be that I missed something, but after MUCH searching, I concluded MF no longer supplied the original cylinders, nor a rebuild kit for them. Several other owners of the semi-orphan 240 reported having the same problem, and having to make the necessary modifications to use the only commercially available cylinders. David, you are correct--they are Sparex. And for whatever reason, I am now sure that they completely escaped the plating process. They will rust from high humidity--they don't have to actually get wet.
But this story looks as if it will have the most unlikely of all good endings. Not thinking that there was a snowballs' chance in Hades (nor did I think I should deserve one, after two-plus years) I contacted YT from whom I bought them and asked if they knew of any defective batch of cylinders from two years ago. I just received a reply from YT's "Jeff", who told me the cylinders were not presently in stock, but that YT and his supplier (Sparex, I assume) had offered to warranty them.
Most of us are quick to publish our anger at something, someone, or some business that annoys us. In the spirit of fairness, we should be as vocal when someone goes "above and beyond" to furnish us good customer service. I was certainly not entitled to a replacement after all this time, regardless that the parts had not been used. Jeff, and YT, I am very much impressed, and very grateful! Thanks!
AND, while I'm gushing, there are a number of folks on this forum who bestow their invaluable experience to those of us who desperately need it. I wish I could recall all your names, but David (from Wales) I have watched as you spent untold time tutoring even the rankest newbies among us, with the patience of Job. You have done that for me in the past. Thank you, David, and all the rest of you who unselfishly give your time to help your fellow man. David, should the time ever arise that I choose to row across the pond (or fly), it would be great to meet you.
Thanks again for all the replies. Perhaps someday I can repay you in kind.
Eastern NC, USA