Tachometer Drive SEAL -- CM-109
BMC part number: 1G2398
For all MGA and MGB through 1964
Can substitute gearbox speedometer drive seal
for all MGA and MGB through 1967 BMC part number: 2A3254 or NKC105A
Apparently the original MGA tachometer drive seal was just a fat rubber O-ring, as it only needs to keep oil from crawling up hill out the top of the spindle.
ID: 0.296" - OD: 0.574" - Section: 0.139" Dash number: 203
McMaster-Carr part number: 9452K184 ($3.48 per 100)
The speedometer drive seal is a proper garter spring seal, because it resides below oil level in the gearbox. As so happens, the speedometer drive seal will also work for the tachometer drive. For replacement procedure see GT-106.
Many years ago (mid 80's I suppose) Moss Motors had a cross reference note in the
Engine section of their catalog saying the tach drive seal was not available, so use the speedo drive seal in its place. No problem. Now they seem to have improvised some sort of substitute seal for the tach drive with a different part number that is $11.85 as opposed to the nice speedo drive seal that is $4.35.
Most of the vendors do refer you to the gearbox garter spring seal for use in the engine tachometer drive. - See list at GT-106A
Apparently the original MGA tachometer drive seal was just a fat rubber O-ring, as it only needs to keep oil from crawling up hill out the top of the spindle. 
ID: 0.296" - OD: 0.574" - Section: 0.139" 
Engine section of their catalog saying the tach drive seal was not available, so use the speedo drive seal in its place. No problem. Now they seem to have improvised some sort of substitute seal for the tach drive with a different part number that is $11.85 as opposed to the nice speedo drive seal that is $4.35.