Since it appeared in the official Gerätliste, the MP40/I cannot be classified as a prototype. Despite its limited production, the weapon was formally approved by the Heereswaffenamt (Army Weapons Office) and deployed for operational use.

It is believed that the MP40/I's dual-magazine feeding system was developed by Erma in 1942 as a response to a Wehrmacht request for a weapon with increased magazine capacity. This initiative aimed to counter the high-capacity drum magazine of the Soviet PPSh-41, which posed a significant challenge in close-quarters combat.

A few differences from the regular MP40 can be noted:

The first and least visible difference is the “cut-out” in the tube or upper receiver. On the regular MP40, there is only a square well for the magazine to enter the tube. The MP40/I, however, has a larger well that almost cuts the tube in half lengthwise. As a result, the bolt “floats” nearly completely free in the tube. This larger well significantly impacts the structural integrity of the tube, and by extension, the entire weapon, increasing the likelihood of breaking or bending. To address this, the top part of the MP40/I’s magazine holder functions as part of the frame for added strength. In photos, it is clear that the steel plate is almost twice as thick as that of a regular MP40, resembling the quality of the pre-war MP38. The oversized cut-out was necessary to accommodate the sideways movement of the magazine holder.

The second main difference is the large magazine holder, which accommodates two magazines. This holder consists of two main components: the top part, which serves as a “rail,” and the lower part, which holds the magazines. The lower section of the holder can be moved sideways by pressing a release button located at the front of the magazine holder. Two magazine release buttons are positioned at the back of the holder. Notably, the magazine holders feature two round cut-outs on the sides to reduce weight, similar to the MP38.

Another difference is that some MP40/I models have a shortened shoulder plate on the folding stock. The double magazine feeding system is positioned slightly further back than the standard feeding system, requiring the shoulder plate to be shortened to allow the stock to fold properly.

Additionally, the ejector has been shortened to match the shorter housing. One observed MP40/I even has the last two digits of its serial number marked on it.

Production Insights

In my view, the MP40/I was not produced as part of a separate manufacturing run. It seems unlikely that these weapons would have been assembled in two different factories. Based on the serial numbers of known MP40/I specimens, it is plausible that a specific batch of MP40s was recalled and modified rather than produced anew. If the MP40/I were fabricated as a separate production run, the numbers would suggest a much larger output than seems reasonable.

Additionally, it is worth noting that all "Steyr-produced" MP40/I models appear to have the pistol grip without the rim, a distinctive feature. This consistency suggests deliberate modification rather than new production.