That IS pretty interesting...
I saw from your post on the NCRS Discussion Board that Terry McManmon came back with an answer referring to a Corvette with VIN #385## that was equipped with 840 heads. The car I saw in Joplin, MO (actually, Terry saw this one too) was VIN #386## and also had 840 heads. These are both mid-December 1969 cars so it looks as though the 840 heads were used for at least one car line until the end of 1969 production.
Your theory that 1.88" exhaust valves were used in 840 heads (or heads used late in the 1969 calendar year) is going to be harder to prove. You're going to need to find cars with original engines to prove this and the owners of these cars are typically reluctant to start taking them apart so someone can check valve sizes
. Add to that the published information from GM that calls for 1.72" exhaust valves and it appears that you have your work cut out for you on this one.
Having said that, I think you may be on to something here. If the 291s were getting 1.88" exhaust valves late in 1969, it makes sense that GM would make it an across the board move and use them in 840s as well... Proving it is going to be the tough part. I'll keep researching this (as well as 840/291 usage) and keep you informed as to what I discover.
Regards,
Stan