Pete E. wrote:

Machine and its case looks to be in really nice shape...unfortunately the machine lost its back-panel sometime during its service life.
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https://shopgoodwill.com/item/256940665
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This machine has a 4-series body and never had a back panel. The paper table and carriage return arm are also 4-series. It looks like one of those odd transition S-C's you some times see with the 5-series keys and paint on 4-series body. If that is a serial number on the receipt, that would be weird too, as I don't know what that 4AR prefix would denote. This may have been a 4-series factory rebuild with updated parts, or some sort of Sears specific machine. If I had to, I would guess the former.

Pete E. wrote:

Maybe the 4AR was a mistake and it really should have been a 4A2...when written on the sales receipt ???
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I think the 4AR is the correct serial prefix. I just don't know what it denotes and I don't think this prefix is currently referenced in any serial lists. This combo of parts/color/body style is uncommon. I have seen it online a few times and there is another example of a 4AR in the Typewriter Database, which also combines a 4-series body with 5-series keys here.

I found a Reddit thread where someone was trying to figure out what was up with these odd 4/5-series crossover models here. In the thread, someone posted this ad from a 1950's Sears catalog showing a 4-series-body/ 5-series-key Smith Corona typewriter. The ad mentions selling discontinued models; so, another explanation could be that Smith-Corona used discontinued parts from the 4-series combined with some updates from the 5-series specifically for an affordable Sears model.