What brand of arched MSH can be "dropped-in" to a Baer frame w/out frame alteration?

I'm not sure if this is a trick question or not....any MSH for a standard 1911, right?

Any "well made" MSH for a standard 1911. My experience with Masen has not been good. Loose fit in frame, mislocated MS retainer pin hole. Some of the S&A MSHs I've seen have been too narrow, though they are usually OK.

Okay ... okay ... I want to stir, too. I just bought, and installed 2 (count 'em, 2) S&A arched mag guides in 2 Colts.
Polished them with Flitz, changed main springs and put them in. Do the words "fit like a glove" sound familiar?
HTH
Walt

Who makes good serrated steel ones...

like the original series 70 Colts? Or am I best off just trying to find a steel series 70 arched, serrated (not checkered) mainspring?

I'm thinking about putting one on a new blue 1991A1.

When I first got my Premier II, I had the same question. I ordered the Rock River arched serrated mainspring housing, because these two brands look so strikingly similar, I figured it would likely be the best fit. About $23.50 or so.

Unfortunately, I still can't answer the question.

I don't know if you've heard about the "RR wait" but it's been about three and a half months, and I'm still waiting on the part.

I called once to confirm that they really had my order, and she said they had run out of these and were going to be making up a batch "soon". Soon in RR time, that is...

In general, none should require frame alteration. But it is likely that any brand will require some filing to the Mainspring Housing, in order to make it fit right.

I seem to recall you can still order the steel Colt ones from Brownells. That's probably what I'll try now.

put an aluminum s&a with mag well in a colt defender and it fit perfectly!!