When you say these were granted on 1/10/20 and transferred on 12/30/20, do you mean stocks that were no longer subject to substantial risk of forfeiture were transferred to your clients on 12/30/20?

Got a few issues here:

1. This is a privately-held company, has the company done some sort of valuation for the stocks?
2. If these are ISOs, §83(b) is not applicable.  Even if the employer's stocks have publicly traded options, those granted by employers are subject to vastly different terms and are not equivalent to publicly traded options.  Stock options granted as part of an equity compensation plan by employers technically do not have any ascertainable FMV and, therefore, do not constitute property transferred in connection with the performance of service for purposes of §83(b).