Penril Corp., a Rockville Manufacturer of data communications equipment, electronic instruments, power supplies and loudspeaker systems, has become in recent months one of the most rapidly expanding companies in the area's technology industry.

In the past 48 hours alone, Penril has signed two major acquisition agreements, and the deals signed on Thursday and yesterday come on top of four earlier acquisitions since 1973. Penril also is preparing applications for listing on the American Stock Exchange; its shares now are traded over-the-counter.

Within three years, Penril President Kenneth Miller forecast in an interview, his firm's annual sales volume will reach $100 million compared with $4 million as recently as 1974 and a current annual rate of about $25 million.

"Penril is an ambitious company . . .

It has been a tough but rewarding road for our shareholders and for our management . . . but we are now at the point where we can take advantage of the opportunities before us in quantities greater than we have ever had before," he said recently.

A key to Miller's plans for Penril, now in its 11th year, is growth by ecquisition in lines of business where the company already has expertise. In the two meals announced this week:

For $3.5 million Penril purchased substantially all assets of Ambac Industries Inc.'s Tele-Dynamics data communication product line, including a series of devices with annual revenues of about $2 million. The Tele-Dynamics operation now in Fort Washington, Pa., will be relocated to Penril's data communications factory in Rockville, where employment will double later this year from about 125 to 250 persons.

For $1.5 million, Penril agreed to buy the Variac auto transformer and line voltage regulator business of Gen-Rad Inc., of Concord, Mass. This accord, still in preliminary stages, also includes licensing of the Variac trade name in North and South America, Australia, New Zealand and parts of Asia.Production would be shifted to a Penril factory in Danbury, Conn., next year; annual revenues of the products involved are about $3.5 million.

Both of these product lines fit in with existing Penril operations and help to demonstrate Miller's strategy of buying divisions of larger corporations rather than small business run by owner-founders, "who have a great deal of emotion attached to their business" and who often retire soon after a deal is completed. In deals with the larger firms, managers of the divisions continue in thier jobs.

Penril also emphasizes decentralized management. A tiny corporate staff in Rockville (seven persons, recently increased from three) overseas the corporation while heads of various divisions have operational autonomy as well as unusual inducements to help the company grow.

Miller said divisional managers' income reflects directly new business they add through acquisition which they identity, "They are helping to get themselves a bigger empire . . . we have the capital . . . we ask them to develop a shopping list, tell them they are on the ground floor of a young company that is acquisition-minded," he stated.

More than 250 large corporations and government agencies use Penril products. A major product line -- one in which Penril is among the top producers for small companies -- is a communications instrument called moden, which converts digital computer language into a format for electronic transmission over telephone lines.

Penril also makes test instruments and working tools for engineering laboratories, factories and to measure characteristics of electronic equipment. Epicure Products Inc., a loudspeaker producer, was purchased for $3 million last November.Currently Penril has 6000 employes in plants at Rockville, Danbury, Amsterdam, N.Y., Newberryport, Mass., and Santa Ana, Calif.

In the fiscal year that ended July 31, Miller forecast that profits rose about 16 percent to $1.05 a share compared with 91 cents a share the previous year. Sales were up about 57 percent to $22 million, he said. Working capital was up 82 percent to $6.8 million and Penril's backlog of contracts was $6.5 million at the end of May, up 26 percent from a year earlier.