yes
Webco and
protec and
white brothers did make them. so they
are period accessoies. i have some. also i got some NOS white brothers ones still in packet
look in the 79 webco catalogue or any 80s WB catalogue and you will see them listed. they are air chamber caps that allow you to increase fluid level. Bob hannah used them. look in a VMX mag from this year from a couple issues ago and the article shows his genuine works yamaha and you will see them on his works KYB forks. also see it here
http://www.mxworksbike.com/1979%20Yamaha%20OW40.htmthe Webco catalogue says.....
These are aftermarket/accessory large air chamber fork caps with extra volume that increases air and oil capacity for better control and promotes extended fade free operation. Fitted with air valves to permit pressurizing front suspension.
and these were also sold for the XL hondas with 23 inch front wheel so when you convert to 21 inch the longer caps allow you to slide the forks out further to restore correct geometry. also helpfull in this case on the RM 125 to extend forks out longer an inch so the forks match the 1 inch longer airshox as Caps is wishing to do.
here are some Protec ones on a DR400
http://www.vintagefactory.com/DR12.jpg