Interesting when 9V applied directly across the mic! Currents and derived resistances below in italic.
- Ericsson round-back N7735A
22mA at 5.3V = 241 Ohms.
200mA at 9V = 45 Ohms.
- Red-front BPO Inset No:13
20mA at 3.4V = 170 Ohms.
100mA at 9V = 90 Ohms.
- Black round-back MOD field telephone No:9 (like yours, probably)
32mA at 1.1V = 34 Ohms.
210mA at 9V = 42 Ohms.
- Red front Ericsson modern N7752
18mA at 3.4V = 189 Ohms.
60mA at 9V = 150 Ohms.
- 'New' BPO type, as fitted to older 706s, stud on spring disc
20mA at 3V = 150 Ohms.
70mA at 9V = 129 Ohms.
Depends on position, I suppose, as the resistance changes with the granule movement. The book ('The Telephone Handbook', Poole) says '... between 30 and 80 Ohms...'