Bias adjustment in Cary SLI-50 integrated amp

I purchased a Cary SLI-50 integrated amp in 1997 and recently replaced the output EL-34 tubes, which requires bias adjustment. The amp has two bias adjustment jacks but does not have adjustemnt screws, like the latest model - SLI-80. I don't know how to make bias adjustment. I can insert a multimeter but cannot make an adjustment, because there are two grey round-shape plugs each located near the respective bias adjustment jack instead of adjustemnt screws.
Can anybody advise me how to make an adjustment?

Best regards,
Alexander