[maple_leaf_eh]

Agreed Interstate are good to deal with (my place to go in uniform and save 20-30%).

A google search says Group 51R for the M151, which I think is the same as the M38A1s. Centre ports to top up the fluid, and both posts on the front corners. There is a dog-bone connector between the two batteries.

More Googling:

If you are not bothered about originality, then go for two good-quality 12V auto batteries with "round-post" terminals. The original batteries are rated at 45-amp-hours.

Just make sure that the positive terminal (+) of the battery is in the bottom right-hand corner of the battery and the negative terminal (-) of the battery is in the bottom left-hand corner of the battery.This is known as Terminal Placement (pole position) O.

Whatever you buy, make sure that the size of the battery does not exceed the following:-

Length - 10 and a 1/4 inches 260 mm
Width - 5 and 5/16 inches 135 mm
Height - 9 inches (to top of terminal lugs) 230 mm

For the USA, there is a very similar civilian battery which is a "22NF", which was used on the Triumph TR-7 (78-82).

NAPA battery 5022NF measures:-
Length - 9 and 1/2 inches 241 mm
Width - 5 and 1/2 inches 140 mm
Height - 9 and 1/16 inches 245 mm
As you can see, the height of this 5022NF is slightly taller than the original battery, so you may have to use a foam insulator for the battery-box lid.

NAPA also sells a battery for the 1993 Honda Civic Del Sol (5051R)
NAPA battery 5051R measures:-
Length - 9 and 5/16 inches 237 mm
Width - 4 and 15/16 inches 126 mm
Height - 8 and 11/16 inches 221 mm