Trying to identify rear english axle on birkin S3

[natanDC]

Hey everyone,

I recently bought a Birkin S3 that's running a Honda CBR1100XX engine, and I'm currently trying to identify the rear axle.

Based on my research, it appears to be a Ford English axle (banjo style). The casting number on the housing is: 73AG 4025 CA T30 — although I’m not 100% sure about the last digit, as it's hard to read.

Here’s where things get confusing:

A typical English axle is supposed to take about 1.1L of oil.

When I filled mine up to the fill hole, it took 3 liters before it started overflowing.

This made me question whether it’s actually an English axle or if it’s been modified in some way.

Also, a few odd details:

The fill plug is on the side of the housing (highlighted in red) — different from most English axles I’ve seen, which usually have the fill plug on the front.

The diff housing appears larger than standard (highlighted in green in my reference images).

Has anyone seen a setup like this before? Could it be a modified English axle or something entirely different?

Appreciate any insights or thoughts!

[Miniliteman]

It's an English axle with modified (enlarged) diff housing. Sometimes used for stock car racing.