Yes, a product declared cash register system without a customer display may be used before 1st of January 2019.
The Bookkeeping Regulation section 5-3-2 states:
The cash register shall be capable of printing sales documentation (receipts) for the customer for each sale, and, unless this proves impractical, the amount registered shall be clearly visible to the customer. The receipt shall always be printed when the amount registered in the cash register is not clearly visible.
From the 1st of January 2017 it is only permitted to sell cash register systems which meet the demands in the new Cash Register Systems Act and Regulations. Enterprises with a bookkeeping obligation have until the 1st of January 2019 to provide such a system, or upgrade the system they already are using.
The Cash Register Systems Regulation section 2-6 (4) states that is shall not be possible to finalize a sale on a cash register system without the system printing out a sales receipt. The motivation for the mentioned rules in the Bookkeeping Regulation will then be fulfilled. The Directorate of Taxes assumes therefore that a product declared cash register system also will fulfill the demands in the Bookkeeping Regulation section 5-3-2a (1) ref. 5-3-2. If the cash register system does have a customer display, the amounts shall be easily visible to the customer. Also referring to the Bookkeeping Regulation section 5-3-12 (1) (comes to force the 1st of January 2019):
The cash register system shall print a sales document (sales receipt) for the customer for each individual sale and the amount registered on cash register systems with customer displays shall be easily visible for the customer unless proved highly unpractical.