Windows 3.1 RC1 is a release candidate version of Windows 3.1, that was shipped to testers on February 14th, 1992, as an early test of the final code.
This version is also mentioned in the Windows 3.1 Program Review from February 27th, 1992. According to the document, the following bugs were reported for this version:
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Internal Test status for AB Apps
These are the results from \\pryex\user—testing\results\isvapps\current\ataged.xls. A * after the bug # indicates the problem also occurs under 3.0.
A LIST APPLICATIONS :
App Name # alt #Pass Bld
ATM 1.10 100% 100% rc1
ATM 2.0 100% 100% rc1
B LIST APPLICATIONS:App Name #alt #Pass Bld
ABC Flow Chart 100% 100% rc1
Actor Pro 3.1 100% 100% rc1
Actor Pro 4.0 100% 100% rc1
Crystal 100% 100% rc1
Freelance 100% 100% rc1 13999 PP: Tools,user setup dialog, minor repaint problems
13997 MF: Arrange,align misusing graphics
13941 MF: Problems rotating TT fonts uses ATM apis
Grammatik 2.0 100% 100% rc1
OBJ Vision 2.0 100% 99% rc1 9680 MF: Repaint problem in hangman sample
9907 MF: Prints TT fonts 1-2 pts too small
SprBase 4(Lan) 100% 100% rc1
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All release candidate builds in that document are simply referred to as "RCX" and not by their actual hardcoded version number. A file from build 101 is versioned "3.1.0.101 (RC1)", so the RC1 referred to in that document must be a later build than 101, or at least a recompile of it.