I recently received a MM RD100 head with good working reverb. I pulled the tank to get the number. It is an
Accutronics short (9-1/4") 3-spring tank 8FB2A1B
1925 Ohms input
2575 Ohms output
Both input and output jacks are grounded.
This tank is available from https://amprepairparts.com/reverb.htm#8.
There are other compatible tanks that have the input NOT grounded but the output grounded. This would require simply running a wire across from the ground of the input to the ground of the output. Those are:
The MOD tanks are available in those configurations and are sold by Antique Electronics, Amazon, eBay, CE Distribution, etc.
Hope this helps.