I operate under the assumption that 1.21 and 1.22 are dramatic updates of the board over a 1.8.
I can't begin to fathom ASUS's janky numbering system back then, but the 1.21 and 1.22 boards seem to have the later more desirable chipsets, which is enough for me to believe they are revisions of the board that were being sold or sent as warranty replacements through the end of 1996 into early 97.
The presence of the SMC disable jumper is likely not indicative of board age. The chip may simply have stopped being available for a period, so they used something else and didn't populate the jumper as it would have undesired effects with the different chip. The SMC chip provides floppy, parallel, both serial ports and basic IDE capability, but I don't think the IDE portion is being used or if it is, it's this chip handling interrupts while data is being processed by the SiS bridge, perhaps using the VLB. This may also be a reason the disable jumper exists - this chip does exactly what a full blown VLB multi-I/O IDE and floppy card did, and it would massively conflict with one should someone want to install one for whatever reason. If a third party expansion card gets installed in the system which has parallel, serial etc, you would need to disable the onboard SuperIO in order for that card's ports to be BIOS supported.