First of all, the maker of your TOPPER shotgun was Harrington & Richardson Arms Incorporated of Gardner,MA. The TOPPER name started in 1943. The Topper model 058 (not just 58) was introduced in 1974 and the Model 490 was made from 1961 to 1983. I can find no published serial number-year made tables for these guns. The original maker went out of business in 1986. According to the Blue Book of Gun Values, your H & R Topper Model 490 JR (a single shot break open breech loader) was made in either 20 gauge or .410 bore. Depending on which you have, use the correct gauge or bore size ammunition. the correct shot size will depend on what you are shooting. Smaller shot for birds, larger shot for rabbits, squirrel, and medium shot for trap and skeet shooting. The larger or higher the number of the shot size, the smaller the shot is.

 

The AL letters in the serial number indicate your H&R Model 490 was made in 1973.

H&R used single prefix letters in the serial numbers (A, B, C) from 1940 to 1963.
In 1964 they started using two letter prefix codes (AA, AB, AC) ending with AZ in 1983. In 1984 the date code went to BA, BB, etc.

But, they skipped the letters I, O, Q, and V in the double letter codes, since these might be interpretted as 1s, 0s, or U instead of V.
That means you can't always rely on the date letters to be a logical progression, since some letters were skipped.

But 1973 is the manufacture year for date code ALxxxxxx.

 

I also have one, where would I see the Serial Number?

 

On all that I've seen, it's on the left side of the receiver, right under the Harrington & Richardson name rollmarks.

So, on the opposite side of the gun as pictured below:

 

Funny I’m seeing this on Feb 27, 2023! I entered “Topper JR MODEL 490” in Google, and this old thread came up. Mine is an “AL” serial number, so apparently it was made in 1973, so it’s 50 years old this year. It’s the only gun I’ve ever owned, and my parents bought it for me back in the day for Christmas. This thing has barely been shot, and it’s in pristine condition. The irony of the Jr model is it’s shorter, which causes more of a “kick” when you fire it. This one is a 20 GA., but it feels more like a 12 GA when you fire it, not that I’ve fired it in the last 20 years. I’m wondering if shells get old? I found shells, but they have to be 30 years old or older. I guess there’s one way to find out.

 

Mine is AU so it would be 1980’s 44 years old!! Mind you I’m 18 and it’s almost older than my mom but how would one find out what shells to use as mines just marked .410 but there isn’t 2 1/2 or 2 3/4 or 3in so how would one find out also I’ve got slugs but could I fire them out of the H&R as it has a modded choke correction I rechecked and it’s pre 1964 so 59yo

 

In my experience, they've always been able to take a full shell, so 3" and anything under as well. Should be marked also.
.410 is pretty small, so you really want as much shot coming out the bore as possible.