"AE" prefix indicates 1968 Value depends upon condition; fair - $50 to Excellent $165
Your harrington and richardson model 900 was made between the years 1962-1973.
25-75 USD or so
No such maker. However, there is a firm called Harrington and Richardson and they did make pistols in 38.
1971
50-100 USD
Blue Book of Gun Values will help.
10-100 USD or so
"P" prefix indicates 1953
50-125 USD
Trademark of Harrington & Richardson Arms company
0-100 USD or so
50-125 USD or so