All Newton devices had at least one PCMCIA memory card slot. According to Engineering360 (the world’s largest online destination for engineers):
“PCMCIA memory cards and storage cards are used to add memory (RAM, SRAM, Flash, etc.) and/or storage capacity (hard disks, CD-ROM, etc.) to computers. PCMCIA is an acronym for the Personal Computer Memory Card Association, the organization which develops and maintains standards for PCMCIA cards. Originally, these devices were known as PC cards because they were designed to add memory to portable computers.”
The original MessagePad and the 100 series Newtons (100, 110, 120, 130) had one Type II card slot, the MessagePad 2000 and 2100 had two Type II card slots, and the eMate 300 had one Type III card slot.
This Newton 4MB Flash Storage Card added 4MB of internal storage to a Newton device.
Sources: Apple Newton UK, Engineering360